Video modem.

(74) Agent: OROSSMAN, Bany, L. et al .; Banner &. Allegretti, Lid., Llth (loor, 1001 G Street, NW, Washington, DC 20001-4597 (US).

(81) Dedgnatad Statar AL, AM, AT, AU, BB, BG, BR, BY, CA, CK.CN.CZ, DB. DK, EE, ES, FL GB. GE, HU, IS, JP, KE, KG. KP. KR, KZ, LK. LR, LS. LT. LU. LV, MD, MG. MK, MN, MW, MX. NO, NZ, PL. FT, RO. RU, SD, SE, SG. SI, SK, TJ, TM, TT, UA, UC, UZ, VN, European patent (AT, BE, CH, DE, DK, ES, FR, GB, GR, IE, IT, LU, MC, NL, FT , I KNOW). OAPI patent (BF. BJ, CF, CG, CI, CM, GA, GN, ML, MR, NE. SN. TD. TG), ARIPO patent (KE. LS. MW, SD, SZ, UG).

Publiibed

With nitrnaticnal search report.

(54) Tltte: VIDEO MODEM

<img file="MX9704045A_D0001.tif" />

(57) Abatract

An apparetui and method is provided fot modulating and tranunitting fuil-motion, television quality color video signáis along with digital data signáis over a pair of onllnary onahlelded twiited pair telephone vires (UTP) without interfering with normal telepbone data on the vires. The invention is characterixed by a tnuumiaiion method involving frequency modulation (VCOl) of a bascband video signa! and subsequent fihering (BPFI) ta auppress aa upper sideband coneapooding to a color cosnpooent of the origina] slgnal. The filtered signal is received ftcm the ilephone vires at a differcnt location, filtered, demodulated and provided to a display device. Full-duplex modulator »over the same pair of vires Is possible, such that tvo video signala raay be atontltaneausly tnnamitted» each signa) having an approximste bandwidth of 6 MHz and shiAed to a deitrable non-intrrfering frequency location whhln the approütimatety 20 MHz of useable bandwidth on the telephone vires. No pre-emphasis or de-emphasis is required to achieve good quality video trenstniuian.

VIDEO MODEM

Background of the Invention

one. Technical Field

This invention relates generally to the transmission of full-motion, television-quality color video signals and associated data and audio signals over an ordinary unshielded, twisted double conductor (UTP) telephone cable, and more In particular, to an apparatus and a method for the bidirectional transmission of these signals on a telephone cable, without interfering with the normal telephone signals that are on the cable.

Related information

Twisted and unprotected (UTP) dual conductor cables are used in many office buildings to transmit voice-grade signals and low-speed data, such as those generated by modems. These cables normally run between 20 and 2_F 000 feet from a central switch or entry point to a home phone. A private telephone exchange (PBX) is normally used to connect the building's wiring network to an external set of telephone lines provided by a telephone company.

Beyond the office building, telephone companies typically restrict the bandwidth of a particular outside ”line to no more than 4 KHz, greatly impeding the ability to transmit, over long distances, other data than width data. very low band. As is evident, a typical NTSC television signal requires approximately 6 MHz of bandwidth and cannot be transmitted directly over these lines. Some companies have attempted to compress or manipulate various types of video signals to fit within the limited bandwidth of telephone cables. Representative examples of this system are disclosed in US Patent No.

5,164,980 to Bush et al. (video telephone system).

However, within an office building, telephone cabling is not artificially limited in bandwidth, but it suffers from the inherent transmission quality of cabling, such as phase shifts and attenuations at higher frequencies. In this way, significantly higher data transmission bandwidths can be achieved within an office building or other structures that have similar wiring arrangements. However, wireline transmission effects can cause drastic changes in received waveforms. These effects, while quite tolerable in voice communications, destroy the integrity of the individual pulses of a baseband video signal. For example, trying to directly transmit a baseband NTSC video signal over a UTP cable, in a building, would result in something totally unacceptable (and also interfere with normal phone data). Amplitude modulation of that signal (as is done in TV transmitters) would also result in something unacceptable. As shown in Table 1 below, the typical loss of attenuation as a function of frequency across 2,000 feet of a UTP cable is severe:

TABLE 1

<td>I FREQUENCY (MHz)</td><td>LOSS (dB)</td>

<td>⁵</td><td> 38</td>

<td> 1 ¹⁰</td><td>⁵⁸</td>

<td> 15</td><td>⁷²</td>

<td> 17</td><td> 76</td>

<td> 20</td><td>⁸² 11</td>

Various schemes have been developed in order to overcome the above and other problems. For example, some schemes involve separating various components of an NTSC signal (such as a luminance signal and a chrominance signal} and modulating and transmitting them separately. An example of this scheme is illustrated in US Patent 4,955,048 to Iwamura. et al. However these schemes normally require complex and expensive signal processing circuits and can be particularly sensitive to device tolerances.

Video teleconferencing systems are already known, where two-way communication, both visual and audio, between individuals or groups that are in different locations is possible. Videoconferencing systems, however, require expensive equipment, high-bandwidth communication channels, and fiber-optic cabling or other high-bandwidth cabling throughout the entire building. High-quality video conferencing on existing telephone cables to date has been difficult, expensive, or impossible in many cases. For these reasons, there is a need to provide a low cost teleconferencing method and apparatus that is used within buildings and has ordinary telephone wiring, without requiring special wires or cables or expensive transmission equipment. .

Summary of the Invention

This invention solves the aforementioned problems by providing a method and apparatus for economically transmitting full motion television quality color video signals and other signals over ordinary unshielded, twisted double conductor (UTP) telephone cables. Unshielded Twisted Pair). The invention is also characterized by a transmission method that allows two NTSC composite signals, together with data and audio signals, to be transmitted bidirectionally simultaneously over a single pair of cables, without interfering with normal voice and data signals and with other signals transmitted on the same pair of cables.

In several preferred embodiments, an NTSC signal is transmitted using an FM signal that has a low modulation rate and a suppressed upper sideband. In a typical telephone installation, the length of the cable running from the central switch or entry point to the telephone can vary, but is usually between 20 and 2,000 feet. By using frequency modulation, the demodulator can amplify and thus limit the received signal. No adjustments are required to install the demodulator for each cable run.

Other features and advantages of the invention will become apparent from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a frequency spectrum of a composite NTSC television signal. Figure 1 (a) shows the spectrum distribution for an NTSC color signal in the baseband; Figure 1 (b) shows a full amplitude modulated NTSC signal; and Figure 1 (c) shows a vestigial sideband AM modulated signal, the lower image sideband having been substantially removed.

Figure 2 shows a frequency plane that is used in assigning the signals to the frequency bands, in a single telephone grade cable, according to the present invention.

Figure 3 is a simplified block diagram showing the components used to simultaneously transmit and receive two NTSC signals in accordance with the principles of the present invention.

Figure 4 is a more detailed circuit diagram illustrating the specific components that can be used in a transmit / receive circuit.

Figure 5 shows how the data displayed on a computer monitor can be transmitted over telephone lines according to the invention.

Detailed Description of the Preferred Modalities

Figure 1 shows the spectrum of a typical NTSC television signal. As shown in Figure 1), a baseband composite NTSC video signal occupies approximately 4.2 MHz of bandwidth, including the luminance signal L, the color subcarrier CSC, and the color signals C that comprise chrominance information (hue and saturation). An SC sound carrier can also be provided with the video signal to transmit audio information. Although not explicitly shown in Figure l (a), the well-known NTSC signal comprises various other synchronization signals that are needed to reconstruct the original signal at the receiver. The details of the signal structure are set forth in the standards promulgated by the FCC (FCC Rules, Chapter 47 of the Code of Federal Regulations Section 73,699, which is mentioned by reference).

When the baseband NTSC signal is used to modulate the amplitude of a carrier signal, the bandwidth typically doubles to at least 8.4 MHz. As shown in Figure l (b), the amplitude modulation process, using the baseband video signal, it produces a signal that has an upper image sideband UPSB and a lower image sideband LPSB centered around the PC image carrier. The two sidebands in any signal contain all the intelligence necessary to recreate the original information. An ASC audio subcarrier includes two ASB audio sidebands.

As shown in Figure l (c), commercial-type television broadcast stations use vestigial sideband AM transmission. The transmission equipment suppresses the lower image sideband in order to reduce the required bandwidth (hence the name vestigial sideband modulation ”). The lower sideband is practically removed, leaving only a trace in addition to the upper sideband. This allows commercial TV to be broadcast with a 6 MHz channel spacing, including audio carriers and guardbands, so that many TV stations can broadcast simultaneously without interfering with each other.

Bandwidth is much more limited over UTP phone cables. To achieve a transmission distance of 2,000 feet, it is necessary to limit the total transmission bandwidth to less than 20 MHz. As previously noted, attempting to transmit an amplitude modulated video signal (as is done with commercial television transmitters) is not feasible on ordinary telephone cable, due to the severe transmission effects that include distortions that cause unacceptable delay of group. Although the use of phase or frequency modulation instead of amplitude modulation could mitigate some of these effects, the required bandwidth would be prohibitive.

Even with narrow deflection FM, a frequency modulated carrier 10 produces a signal spectrum that is at least twice the baseband frequency. For video signals, this would require a minimum of 10 MHz per channel. For full duplex operation (i.e. simultaneous transmission of video signals in both directions on the same wire or cable), two 10 MHz channels would be required, which would consume virtually all available bandwidth, with no bands guard.

To resolve the above limitations, the

0 This invention contemplates the use of a vestigial sideband FM signal. " This means that one of the FM modulation sidebands is removed in the transmitter, preferably the upper sideband for reasons that are obvious. By using this type of modulation, the baseband signal. Original ntsc can

-1010 can be rebuilt using only 6 MHz of the bandwidth, while allowing a few megahertz to be available for the interchannel guard band. The 6 MHz band can include a broadcast-quality video signal and the accompanying audio signal, although in various modes the audio signal is filtered along with the upper sideband. One or more CD quality audio signals can also be transmitted using a separate data channel.

It should be recognized that the principles of the present invention can also be used with PAL and SECAM television signals, with appropriate modifications in bandwidths and center frequencies to adjust for differences in spectra. For example, instead of a channel bandwidth of 6 MHz, PAL and SECAM systems may employ channel bandwidths of 7 to 8 MHz. The details of the PAL and SECAM signal structure are well known and are not are repeated here. Although AM and narrowband FM have similar frequency spectra, they are distinctively different modulation methods. In the case of AM, the carrier envelope varies, the frequency remains unchanged; in the FM case, the carrier amplitude is assumed constant, the phase (and the instantaneous frequency) varies with the signal. It should be noted that when using an FM modulation index

Low -1111 would apparently eliminate many of the disadvantages of using FM modulation in the first place, but as described here, the resulting signal is adequate for transmitting television-quality images over an ordinary telephone cable, even without using circuitry. traditional accentuation / de-accentuation. The use of a smaller modulation index helps to avoid phase shifts that would occur at higher deviations.

As shown in Figure 2, the present invention contemplates a frequency plane that allows multiple signals to be transmitted simultaneously within the available bandwidth on the UTP cable. In Figure 2, the amplitude is represented on the vertical axis and the frequency in megahertz is represented on the horizontal axis. A number of the signals are assigned in the frequency band ending at approximately 20 MHz, the preferred limit for unprotected telephone cable, as contemplated by the present invention. As described in more detail below, video signals A and B can be used to transmit video-quality images over the cable

UTP.

A first signal T represents the telephone signals existing at the very low end of the spectrum.

-1212

These signals can be analog or digital. In either case, its spectrum components are typically much less than 1 MHz.

Di and D2 data signals can be centered at approximately 1.5 MHz and 3.5 MHz, respectively, and can be used to transmit bidirectional high-speed data over the cable, using any of several different known modulation methods (including PSK modulations, QAM or FSK). Alternatively, the DI and D2 data signals may each comprise an FM signal (FM1 and FM2 respectively) to transmit frequency modulated audio data corresponding to video signals A and B, respectively. In addition, Digl and Dig2 digital data signals represent digitally modulated data streams that may also be accompanied by A and B video signals. In this way, each DI and D2 data signal may comprise any one of several types of signal modulation that can be used to transmit information that is preferably related to corresponding video signals A and B, respectively. The exact frequency placement of the DI and D2 data signals can be varied, consistent with the telephone signal T and the video signals A and B.

A carrier for video transmitting signal A

-1313 is illustrated centered at approximately 9 MHz and a carrier for the video transmitting signal B is illustrated centered at approximately 17 MHz. An outer lower sideband of signal A (corresponding to the color subcarrier) is represented by LSB_TO, while an outer lower sideband of signal B (corresponding to the color subcarrier) is represented by LSBg, The upper sidebands contain the color subcarrier signals that have been suppressed in Figure 2, and therefore not shown. Sound carriers, positioned above the upper colored sidebands, have been removed and are not shown.

According to the frequency plane of Figure 2, the two video signals can be transmitted simultaneously through a single cable, each having a bandwidth of approximately 6 MHz. It should be noted that the illustrated center frequencies of the video and data signals are given only as examples and of course it is possible to move these signals within approximately 20 MHz of the useful bandwidth or even further (if one wishes to accept signals image quality). However, it may be possible to use bandwidths of less than 6 MHz for each video signal, with easily recognizable offsets in

-1414 image quality and the like.

Good image quality on ordinary telephone cables can be obtained using an NTSC video signal to frequency modulate a carrier signal and transmit only the carrier, closing sidebands, and an outer sideband containing the 3.58MHz color subcarrier. , preferably the lower sideband. In one experiment, the carrier signal was centered at approximately 10 MHz, closing the sidebands in the 9-11 MHz range, and the outer lower sideband at 6.42 MHz (i.e. 10 MHz - 3.58 MHz). A SAW filter having a 3 dB bandwidth of 6 MHz was used. This bandpass was translated into frequency to be between 5 and 11 MHz. The lower sideband centered at 6.42 MHz actually has its own side subbands that mimic the shape of the closing sidebands around 10 MHz. Apparently it is important to transmit these sidebands with reasonable fidelity in order to maintain a good image quality. Extending the bandpass filter below 5 MHz (i.e. approximately 1.6 MHz below 6.42 MHz) seems to satisfy this requirement.

When considering single phase modulation of a carrier with a lower modulation index, the effect of suppressing a sideband is to convert the carrier

-1515 purely phase modulated into one that is simultaneously amplitude and phase modulated. If this signal is passed through a limiter at the receiving end in order to suppress amplitude modulation, a pure phase modulation is restored, but with half the modulation index.

By placing the carrier near the top end of the passband, so that the transmitted sideband is the bottom, the effect of increasing the attenuation with frequency on the twisted double conductor cable is to activate the bottom sideband relative to the carrier. This is in the optimal direction to compensate for the reduction in modulation index due to suppression of the upper sideband. Since the sound carrier in each NTSC signal is located in the portion of the spectrum that is cut transmitting only the lower sideband, the audio signal can then be modulated over an FM carrier and transmitted as FM1 or FM2, for example (see Figure 2 ).

In a duplex operation on a UTP, filtering is required to separate the transmitted signal from the much weaker received signal, and some tolerance must be had for the guard or transition bands of the filters used. Even in the case of a SAW filter, the transition band should be approximately 1 MHz wide.

-1616

In various modalities a guard band with an amplitude of 2 MHz has been assumed.

Based on the above conditions, a frequency plane such as that illustrated in Figure 2 is preferred, but it is not intended in any way to limit the principles of this invention to those embodiments. As an example, a proximal transceiver can be placed at the point of the central telephone switch and a distal transceiver at a user terminal, for example an office. The carrier frequency of the neighboring transmitter may be 17 MHz with nominal band limits of 13 to 19 MHz (signal B in Figure 2). The distal transmitter carrier can be 9 MHz, the band limits 5 to 11 MHz (signal A in Figure 2). In this way, the guardband ranges from 11 to 13 MHz. It is a simple task to make minor adjustments to these carrier frequencies to optimize the frequency in any particular application.

In the losses illustrated in Table 1 shown, the predicted loss of 2000 feet from the UTP 3 level will be approximately 76 dB at 17 MHz, but only approximately 56 dB at 9 MHz or 20 dB less. Since there is a need for some minimum carrier-to-noise ratio at the receiver, you want to transmit much more power at 17 MHz than at 9 MHz.

Still another consideration is that the second

-1717 harmonic distortion of the 9 MHz to 18 MHz carrier will have to be vigorously suppressed at the distal station in order to avoid interference with the weak received carrier at 17 MHz. Therefore, it is incidental that the 9 MHz carrier can be relatively weaker. In the case of the 17 MHz transmitter, the harmonic components at 34 MHz and higher will be removed from the pass band of the receiver.

Assuming a noise figure of 10 dB at the receiver, together with a noise bandwidth of 6 MHz, a received signal strength at the distal station of -59 dB should give a video-to-noise signal ratio of approximately 37 dB, which would be adequate for most purposes.

Figure 3 illustrates a component block diagram, used to carry out the principles of the invention between a first transceiver 1 and a second transceiver 2. As illustrated in Figure 3, transceiver 1 accepts the first signal from video V_INI and, after processing it according to the principles of the invention, transmits it over the UTP cable to transceiver 2, which regenerates the original signal and outputs the video signal Vqu ^, for display on a television receiver . Similarly, transceiver 2 accepts a second video signal Vjj ^ 2 as input, and after

-1818 process it according to the principles of the invention, transmit it over the same UTP to transceiver 1, which regenerates the original signal and outputs the video signal Vquti * The input video signals can be generated from a camera, VCR, computer or the like, and the output signals may be provided to a display or recording device such as a video monitor, another VCR or a computer video display.

The UTP cable can also be connected to one or more TEL telephones and to a switching unit (for example a Telephone Exchange or Centrex), so that normal telephone conversations can be done on the same cable while the video signals are being transmitted. It will also be apparent that while bi-directional transmissions with two transmitters are illustrated, it is of course possible that two video signals are transmitted from a first location to a second location, each of the signals using a separate carrier frequency. In that case, two transmitter circuits at one end and two receiver circuits at the other would be required.

Starting with transceiver 1, a first video signal V_{IN 1} Enters the VCO / Amplifier / Mixer VCO1 which has a carrier frequency of, for example, 17 MHz.

-1919

The input video signal includes luminance and chrominance components as known in the art and as illustrated in Figure l (a), and the frequency modulation of this signal will produce upper and lower sidebands, each sideband has a copy ”of the color subcarrier. Although Figure l {b) shows the spectrum of an AM modulated signal, the spectrum of an FM modulated signal will be similar even though the signal is different. As previously observed, the modulated signal on the 17 MHz carrier is preferably amplified relative to the modulated signal on the 9 MHz carrier, in order to compensate for attenuation losses at higher frequencies.

The frequency modulated signal is passed through the BPF1 bandpass filter with a 6 MHz passband, which expands from 13 to 19 MHz, although as in the frequency plane of Figure 2, they are not transmitted For signals above 19 MHz, this filter works to match impedances and to provide suppression of 17 MHz harmonics. The filtered signal enters the balun transformer and the Hl hybrid circuit, which injects the signal to the UTP line through the DFl duplex filter. The purpose of the duplex filter is to inject video modem signals over the UTP while preventing video modem signals from entering the line.

-2020

Since the upper sideband has been removed, the signal injected from H1 has a frequency spectrum that is roughly shown as B, illustrated in Figure 2. Other data signals, such as DI, can be input as illustrated in the frequency of Figure 2 with appropriate modulation. In this way, for example, the Djjjj terminal can be used to accept a data signal (for example an audio signal) that is modulated by the DM1 digital modulator and subsequently passed through the BPF6 filter and injected into the hybrid H1. The recovery of the signal in transceiver 2 is done through the BPF10 filter and the DD2 digital demodulator to output terminal Dq_UT2 Although not explicitly shown in Figure 3, an FM modulator can also be included to transmit frequency modulated audio signals such as FM1 and FM2, illustrated in Figure 2, which correspond to video signals A and B respectively.

Since the components of transceiver 2 on the right side of Figure 2 are similar to those on the left side (but use different frequency bands), the receiver function will be described in relation to transceiver 1, and the corresponding explanation for transceiver 2 will be omit for brevity.

Assuming that transceiver 2 transmits a

-2121 second video signal to transceiver 1 over UTP cable (illustrated by signal A in Figure 2), the signal is received at H1 and applied to the bandpass filter BPF2 which has a preferred bandpass interval 5 to 11 MHz. The filtered signal is applied to the C1 preamplifier and upconverter. In Figure 3, each receiver is of the superheterodyne type, using a local oscillator and mixer for the upconversion of the signal to the 46 MHz center frequency of a SAW filter. This approach allows the available SAW filters to be used and has the additional advantage of allowing the frequency plane to be modified. It should be noted that individualized filter designs can also be used, eliminating the need for frequency conversion.

A SAW BPF3 filter provides an amplified output to the video limiter / discriminator / amplifier L2, the output is further amplified towards the video levels (NTSC, PAL or SECAM) if the recovered video signal is output as V_outl. It has been determined that excellent results are obtained without pre-accentuation or de-emphasis of the signal.

<td></td><td>The</td><td>Figure 4 shows</td><td>a detailed outline</td><td>than</td>

<td colspan="2">use the</td><td>early</td><td>present invention.</td><td>The</td>

<td>circuit</td><td colspan="2">works from</td><td>supplies +5 and +12</td><td>VDC</td>

<td>regulated</td><td></td><td>The VCO transmitter</td><td>U6 is a multivibrator</td><td>with</td>

-2222 a relatively linear control characteristic. In the present invention, the FM deviation is typically large compared to the carrier frequency, so that the moderate offset of the center frequency and the

<td colspan="4">phase noise are of no concern</td><td colspan="2">considerable.</td><td rowspan="2">the</td>

<td>The</td><td>input from</td><td>video</td><td>NTSC</td><td>than</td><td>It includes</td>

<td>subcarrier</td><td>color it</td><td>apply to</td><td>E5</td><td>and ^{I know}</td><td>makes pass</td><td>to</td>

<td>through a</td><td colspan="2">low pass filter</td><td>than</td><td>has</td><td>a cut</td><td>of</td>

about 5 MHz.

The multivibrator output is AC coupled to a section of U4, a separator that amplifies the multivibrator levels from 0 to 5 VDC. The amplified output drives the remaining 5 sections of U4 in parallel.

Since the output of the hexa separator is a square wave with numerous harmonics, it is important to design the following filter so that it has a high input impedance at the frequencies of these harmonics, that is, the first circuit element must be an inductor in series.

In the case of 17 MHz, the output center works primarily to match 7.5 ohms to 50 ohm output impedance and to provide moderate harmonic suppression of 17 MHz. For 9 MHz, the filter should provide the attenuation of the second harmonic at 18 MHz.

-2323

In the case of receive filters, the filters are typically designed for a 50 ohm input impedance and a higher output impedance, for example 400 or 800 ohms. The increase in impedance provides a voltage gain that works within the relatively high input impedance of the U5 frequency converter.

Filters initially designed to pass the 9 MHz carrier were implemented using the ferrite core inductors from a coil case. High and mysteriously variable levels of the second harmonic energy are observed at 18 MHz. These were found to be caused by nonlinear distortion in the ferrite cores, which also depends on the orientation of the cores relative to the Earth's magnetic field, i.e. a flux gate magnetometer had been inadvertently implemented. Therefore, the use of air core solenoids is recommended.

The output of the transmission bandpass filter is applied to a TI transformer that simultaneously acts as a balun and as a 3 dB hybrid. In this transformer, terminals 4 and 7 are the balanced output that drives the UTP with a nominal generating impedance of 100 ohms.

Terminals 5 and 6 of the TI transformer are

-2424 are derived via capacitor Cl, which is a relative short circuit above about 1 MHz. DC signals or voltages below 1 MHz can be made available at terminals AUX1 and AUX2 for use with other equipment. Terminals 1 and 3 are 50 ohm mutually isolated ports of the hybrid. A 25 ohm load on terminal 2 effectively terminates the sum port of the hybrid transformer. Data signals can be coupled using a duplexer filter designed to have a wideband input and two outputs, one corresponding to signals from DC at approximately 4 MHz, and the second to signals from 5 MHz to above 20 MHz.

In the receive path, the signal received outside terminal 1 of the transformer TI is applied to receive the BPF5 filter (BPF2) at the distal transceiver (next!). As previously noted, BPF2 should be designed to provide rejection in the 13-20 MHz band, spanning the 18 MHz harmonica.

The filtered signal is applied to U5, a frequency converter device that combines the functions of a local oscillator and an active mixer. Diodes U3 placed against each other bypass the input to U5 in order to handle strong received signals that may arise when the UTP line is short.

-2525

The U5 output has an impedance generator of approximately 1.5K. A PNP emitter followed by Q1 helps to set this impedance low towards 50 ohm input impedance of stage Q2. The emitter follower Q1 also helps stabilize the DC operating point of the Q1-Q2 cascade against variation in V_BE with temperature. Stage Q2 provides approximately 30 dB of voltage gain along with high output capacity (relative to supply voltage + 5VDC) due to the use of L3 as a collector load. Stage Q2 has a low output impedance as desired to improve the response of the SAW filter.

The SAW U7 filter operates on a balanced load for improved input-output isolation. The small bypass capacitor, C29, slightly improves the shape of the pass band of the SAW filter.

The amplified output of the SAW filter is applied to U10, a limiter-discriminator circuit. In addition to an adjustment of the quadrature coil network for broadband operation at 46 MHz, this circuit provides good limiting action in a 40 dB range, with excellent discriminator linearity.

The U9 broadband splitter and U8 optical amplifier implement a differential input video amplifier to amplify the discriminator output

-2626 towards NTSC video levels. The wave frequency at the output of U10 is approximately 92 MHz. In this way, simple low-pass filtering by C25 and C33 provides adequate rejection of fluctuations in the video output.

Regarding the pre-accentuation of the signal applied to the FM modulator, it is perceived as a distinct advantage of the present invention that pre-accentuation and de-accentuation are not required, although the invention will, of course, work with this type of additional circuitry.

Figure 5 shows how the principles of the present invention can be used to transmit data displayed on a computer monitor. Computer 501 may be a PC type computer having a red, green-blue (RGB) output that is supplied to a scan converter 503 which is well known in the art. The RGB signal from computer 501 is input to the monitor on computer 502 to drive its display. Browser converter 503 converts RGB signals into a video format, for example the NTSC format, and supplies this to video-mobile 504 which is built according to the principles of the present invention and coupled to a UTP telephone line. In this way, the data that is displayed on the monitor

-2727 computer 502 can be transmitted over the UTP phone line and displayed simultaneously on another monitor at the receiving end.

Thus, a method and apparatus for transmitting television quality video signals, as well as other signals, over ordinary telephone grade cables has been described, in accordance with a suggested modulation scheme and suggested frequency plan. The invention has many possible uses and benefits and performs the desired functions with a minimum of circuit components.

The foregoing description and accompanying drawings provide various preferred embodiments of this invention. Anyone with ordinary expertise in this field will understand that specific references to components, which are made here, are given by way of example only, specific devices and device values ​​are mentioned as particular requirements and engineering offsets that are involved in a particular implementation. As an example, it will be appreciated that different cycles and demodulators can be augmented with a digital signal processing device rather than using analog components. It will be apparent that many modifications and variations can be made to this invention in light of the above teachings. For the

Therefore, it will be understood that within the scope of the appended claims variations can be practiced that are not specifically described.

of the others

-2929