24.1.1 Broadcast Versus Broadband Receivers

The technical requirements of tuners for broadcast channels versus broadband cable channels are significantly different. It is possible to design an excellent broadcast tuner that performs poorly on cable channels, and vice versa. Designing a cost-effective tuner that performs well in both environments is a significant challenge.

The broadcast environment consists of channels with gaps. Local channels are almost never in adjacent frequencies. However, the viewer may wish to receive a weak signal with frequency between two strong local signals. The weak signal may be from a distant location. This is called fringe reception and was an important selling feature before cable became popular. The tuner must function well in that environment. Note that the lower channel numbers, the VHF channels, are designated by the FCC so that the frequency of no one channel is a second harmonic of any other. This reduces the requirements on linearity in the tuner circuits.

Broadcast channels are not located on an “image” channel of another broadcast channel. The image channel problem will be described later in this chapter. Image interference problems are avoided in the broadcast environment. An inspection of the frequencies assigned to the VHF channels reveals that no channels are assigned so that they are images of valid channel frequencies. The higher channel numbers, the UHF channels, have an elaborate set of “taboos” in the FCC’s rules that are intended to relax the design requirements of practical consumer electronics tuners. Finally, in the broadcast environment, there is no particular need to prevent direct pickup of the broadcast channel. Direct pickup simply adds to the signal brought in from the antenna and causes no harm. Also, since only broadcast channel frequencies are found in the tuner, there is no need to prevent their radiation from the tuner terminals or from the tuner itself.

The cable tuner is very different. The cable spectrum is fully loaded with no significant gaps up to the highest frequency. The signal levels are strong and relatively uniform across the frequency band. The challenge is in handling a lot of energy and in avoiding distortion and cross modulation of the signals.

A consequence of these differences is that a properly designed cable tuner will probably not perform well in the broadcast environment, and a properly designed broadcast tuner will probably not perform well in the cable environment.

Essentially, all cable converter tuners are double conversion tuners. That is, they apply the superheterodyne principle twice. They have two local oscillators and two intermediate frequency amplifiers and filters. This gives improved selectivity and blocks the image channel. Also, the first local oscillator frequency is usually above the highest channel tuned, and the second local oscillator signal is isolated from the cable by the first conversion stage. So backfeed of internally generated spurious signals is not a problem with cable converters. The use of a double conversion tuner in a “cable-ready” TV or VCR is extremely rare. Nearly all consumer electronics tuners are single conversion devices.

The deficiencies of “cable-ready” tuners will become more apparent as cable systems expand bandwidths beyond 450 MHz on to 750 MHz and even 1.0 GHz and beyond.

24.2 Levels of Interference

Four kinds of interference have been considered. In order of severity, they are (1) interference with the product’s own performance, (2) interference with other products in the same home, (3) interference with another subscriber’s reception, and (4) interference with other users of the electromagnetic spectrum.¹

Interference with the product’s own performance results in a dissatisfied customer for the product and probably for the services delivered. Unfortunately, in many cases, the poor performance of the equipment can be confused with a problem with the service. This results in unnecessary and unproductive service calls, missed work while waiting for the service call, and the potential for finger-pointing between the cable service provider and the retailer of the consumer product. Too often the consumer gives up. The problem is not resolved, and the consumer is suspicious of both the consumer electronics product and the cable operator.

The next level of severity occurs when the product backfeeds interfering signals into the cable, causing problems with another TV or VCR in the same home. This is often a transitory problem occurring only when certain combinations of channels are tuned by the two devices. This is likely to be blamed on the cable operator. Yet one step worse is when another subscriber suffers interference from a product not even in the same home. This will almost always be blamed on the cable operator.

The most severe situation is when interference is caused to other users of the radio frequency spectrum, particularly safety-related users such as aircraft communications and navigation, fire, police, and emergency communications. This level of interference goes beyond merely reducing the entertainment value of cable and can become dangerous to others’ well-being.

24.3.1 Estimate of Exposure

CableLabs and the EIA funded research by consultants to study the problem. Stern Telecommunications Corporation of New York City did a study of the potential impact of DPU. The study included a literature search, measurement of representative recent “cable-ready” receivers, and a computer model of transmitter power levels at various distances from the transmitter.

The receiver testing was done at the Carl T. Jones Corporation in Springfield, Virginia, in July 1992. Five receivers were tested. The testing revealed that there is a range of susceptibility to DPU. This susceptibility is a complex phenomenon that depends on the orientation of the receiver relative to the signal’s field, the way in which the coaxial cable and the power cord are oriented with respect to the signal’s field, and the channel involved.

The computer modeling considered the 10 top broadcast-relevant geographic areas, called areas of dominant influence (ADIs). The ADI terminology is important in considering advertising reach and effectiveness. For the purposes of this study, the ADIs have well-documented data on the distribution and density of households. This geographic data is important to the estimates of exposure to DPU. The 10 ADIs include about 30% of U.S. households. The field strength contours were computed with a commercially available program by Communications Data Service, Inc., which is based on the FCC’s propagation curves. The study considered only UHF channels up to channel 27 (visual carrier at 549.25 MHz) on the assumption that most cable systems are limited to 550 MHz. As such, the study is a little out of date, but the general trend is clear. Field strengths of 80dBu (10 mV/m), 90 dBu (31.6 mV/m), 100dBu (100 mV/m), and 120 dBu (1 V/m) were considered.

Data was presented for each of the 10 top ADIs and extrapolated to all U.S. households. Table 24.1 lists the percentage of all U.S. households that may be subject to at least the field strengths listed from stations operating up to 550 MHz.

Table 24.1 Exposure to various levels of fields

This data shows that the previously used Canadian specification of 100 mV/m will leave a significant number of receivers vulnerable to DPU. Even at an immunity level set at 1 V/m, a modest number of subscribers will still either have to accept DPU or take a set top terminal with better shielding.

24.3.2 Estimate of Susceptibility

In December 1992, CableLabs solicited proposals to develop test methods for measuring DPU susceptibility and to measure a representative grouping of current products used in the home. The Carl T. Jones Corporation was hired for both of these tasks. Thirty-five television receivers, 8 VCRs, and 13 cable settop terminals were tested for DPU susceptibility. The products were subjected to fields of increasing strength until a DPU 55 dB below the desired channel’s sync tip was achieved. The 55-dB figure was confirmed to be reasonable by perception of interference studies. The discussion in Chapter 2 on the W curve and DPU indicated the 1993 tests generally confirmed the results of the 1986 CBS experiments. Across the three channels, four frequencies, two carrier-to-noise ratios, and two desired signal levels, the average value of the sine wave strength attenuation relative to the carrier strength to achieve “just perceptible” impairment levels was 56 dB.

Table 24.2 summarizes the results for the sample set of products. The mV/m figures required to breach the −55-dB threshold for slightly worse than “just perceptible” interference are listed for the best, worst, and median products. As can be seen, for the 1993 model products tested, a wide range of results were obtained.

Table 24.2 Performance of units tested

An important difference between set top terminals supplied by cable operators and consumer electronics products is that the cable technician has several units with him or her and will keep trying them until the DPU problem is solved. Since the consumer purchases a TV or VCR and takes it home, the ability to pick the best from a collection is not available. Also, when the consumer makes a purchase decision, that selection will usually be in a different neighborhood from his or her residence. The retailer may not be using a cable feed for display purposes. Often, a laser disc is used as the source in order to provide the best possible pictures to encourage a purchase.

24.4.1 Reradiation of Cable Signals

Cable systems operate in an enclosed spectral environment. If this environment is sealed, the same frequencies that are used in the over-the-air spectrum for other purposes can be reused for cable channels. In fact, multiple enclosed cable systems can be simultaneously operated to further increase the number of signals delivered to subscribers. As long as the cable system remains sealed, no harm is done to other users of the over-the-air spectrum.

A potential source of signal leakage exists in any equipment connected to the cable system. This is because the entire cable spectrum is conveyed to the input terminals of the connected device. If leakage of these signals occurs because of inadequate shielding of the circuits, the potential for interference with other users of the spectrum exists. This is especially serious in the case of frequencies used for aircraft navigation and communications and for emergency messages. Cable systems occupy these frequencies in their closed systems and cause no harm as long as the integrity of the system remains intact.

Equipment connected to the cable system must also be designed to prevent signal leakage. If this equipment leaks signals, interference may result. Set top terminals purchased by cable operators are specifically designed to prevent such leakage. The output of these set top terminals is just one television channel that is on a frequency not used locally for over-the-air service, usually channel 3 or 4. If leakage of this channel occurs from the TV or VCR connected to it, no harm is done since this is an unused local frequency. Consumer electronics products designed for broadcast applications do not have this requirement. When used for broadcast reception, they do not tune frequencies that are used for other purposes; they just tune the broadcast channels. Because of these differences, it is important to impose additional requirements on products connected to cable and prevent the connection of equipment that is not specifically designed to avoid leakage.

The FCC rules, Part 76.605 (a)12, limit the leakage or egress of RF energy. Cable systems must not emit energy that causes more than 15 μV per meter at a distance of 30 meters for frequencies less than 54 MHz or greater than 216 MHz. Between these frequencies, the limit is 20 μV per meter at 3 meters distance.

Of the units tested, only two television receivers failed.

24.4.2 Local Oscillator Leakage and Backfeed

Television receivers generate internal signals that are used to process the received spectrum. In addition, other internal signals are generated to display information on the screen and to operate functions and features of the product. Switch mode power supplies are also a source of interference. If these signals backfeed into the cable system, they can cause interference and degradation of performance in other TVs and VCRs connected to the cable system. They can add to the background noise, which makes two-way cable operation difficult. The other TVs and VCRs on the same signal splitter in the same residence are most at risk since there is a minimum of attenuation of the interfering signals. However, neighbors’ receivers on the same cable system tap are also subject to this interference. Consequently, signal backfeed is carefully specified in equipment used by the service provider. Consumer equipment requires the same degree of care if problems are to be avoided.

The local oscillator in the single conversion tuners commonly used in TVs and VCRs operates at 45.75 MHz above the desired channel’s visual carrier frequency. This frequency is in the frequency range of the cable system for most of the channels tuned. If this signal leaks back into the cable system, interference will result to other receivers according to the W curve discussed in Chapter 2. Fortunately, the splitters used by cable operators are required to have a minimum of 18-dB isolation between ports and usually have 20 dB or more. Thus if the local oscillator or other signal backfeed is no worse than 35 dB below the visual carrier, the 20 dB of the splitter will increase the signal attenuation to 55 dB below the visual carrier when it reaches the other receiver connected to the same splitter. At this level, it will be “just perceptible,” but not annoying. Any further losses in the connecting cable will also help. Cable set-top terminals avoid this problem by using double conversion tuners, with the first local oscillator frequency higher than the cable spectrum tuned.

24.4.3 A/B Switch Isolation

A/B switches are part of the input of many receivers and VCRs, and allow remote-controlled selection of input sources. These sources include external antennas; cable signals; and the outputs of set top terminals, video games, and other playback devices. If insufficient isolation exists between ports of the switch, cross feed of signals will occur. The FCC regulations dated 1987 in Part 15.606 require a minimum of 80-dB isolation for frequencies between 54 and 216 MHz; 60 dB is required above that to 550 MHz. Since 1987, cable systems have gone beyond 550 MHz. Switches that have inadequate isolation at these higher frequencies will be a source of difficulty.

When an antenna is used, two kinds of problems may result. Antenna input signals can exceed +25 dBmV, whereas cable signals can be as low as 0 dBmV — lower if the subscriber does his or her own wiring and has split the signal excessively without appropriate amplification. In those situations, the isolation must be adequate to prevent interference. This explains why the cable industry uses switches that generally exceed 90 dB and often 100 dB at all used frequencies. The other problem arises when strong cable signals are not sufficiently isolated from a rooftop antenna. If cross feed of the cable signal occurred in many homes, the cumulative effect of efficient radiators all pointing in the same direction, that is, toward the broadcaster’s transmitting tower, could yield unacceptable interference to aircraft traversing that path.

Although most of the devices tested had no problems with this specification, there were three instances that failed.

24.4.4 DPU Backfeed

The DPU backfeed issue is related to the DPU phenomenon but is not the same problem. A receiver that has a DPU problem may also backfeed such signals into the cable to cause difficulties with other devices. Again, there is a benefit from the 20-dB isolation between signal splitter ports. DPU backfeed of less than −35 dB will be at the perceptible but not annoying point.

Unfortunately, 37% of TVs had DPU backfeed above the perceptibility point on channel 6, 34% on channel 12, 26% on channel 78, and 17% on channel 59. Understandably, units with DPU problems had DPU backfeed problems as well.

24.4.5 VCR Through-Loss

Designing VCRs with adequate playback signal quality is a difficult technical challenge. A technique used in early VCRs to mitigate the problem was to unsymmetrically split the signal at the input terminals of the device, giving the larger share to the recording circuits and shortchanging the “through” side of the splitter. The through side of the splitter provides an attenuated version of the entire input spectrum to the RF output terminal of the VCR when neither playback nor the VCR tuner is used. The attenuation is due to the use of a splitter without first amplifying the signal. This trick helped the noise performance of the recording circuits while providing a less disadvantageous comparison for the viewer when watching unrecorded materials.

As the last chapter discussed, the most common configuration of set-top terminal, VCR, and TV connects the cable system to the set-top terminal, which is connected to the VCR, which is connected to the TV. The TV suffers the additional through-loss of the VCR. This through-loss should be no greater than 5 dB. All eight tested VCRs passed this hurdle.

24.4.6 Adjacent Channel Rejection

The broadcast environment avoids adjacent channels. Cable utilizes as much of the spectrum as possible with nearly all channels having adjacent neighbors. An important issue for cable tuners is the ability to separate the desired channel from the upper adjacent visual carrier and the lower adjacent aural and chroma carriers. Cable practice is to operate the aural carriers some 17 dB below the visual carriers. This additional attenuation relaxes the difficulty somewhat. The criteria again is the 55 dB used to bring the signal to the not-annoying level.

The passband of the receiver is a combination of the IF response and the tuner response, which differs slightly from channel to channel and is under the influence of the automatic gain control (AGC) strategy. Consequently, devices may have isolated points of failure on some channels but not on others.

Of the devices tested, three television receivers and one VCR failed to sufficiently reject the lower adjacent color subcarrier. Two television receivers failed the aural rejection test on all channels, and two others had problems on just two channels. Of the eight VCRs, three had no adjacent aural problems, three others failed on the four channels tested, one failed on three channels, and one unit had difficulties with just one channel. All TVs and VCRs behaved adequately with the upper channel’s visual carrier. Not surprisingly, cable set-top terminals passed all tests since they are specifically designed for such service.

24.4.7 Image Rejection

Image channels are a result of the superheterodyne receiver design, which uses a balanced mixer with two inputs followed by a filter for the purpose of separating out the desired signal from the spectrum at its input terminals. The inputs to the balanced mixer are the received spectrum and a local oscillator. The output of the mixer consists of the sum and differences of the input frequencies.

TV local oscillators for VHF are 45.75 MHz above the desired visual carrier. The difference between the local oscillator frequency and the desired frequency is the intermediate frequency (IF), which passes through the filter characteristic of the IF amplifier. The signal is then detected and becomes the source for the picture and sound selected by the viewer. The sum of the local oscillator and the desired channel is blocked by the filtering of the IF amplifier.

There is another difference frequency equal to 45.75 MHz. Signals that are 45.75 MHz above the local oscillator frequency will also have a difference of 45.75 MHz relative to the local oscillator frequency. This signal, if present at the input of the mixer, will be heterodyned to a frequency that will pass through the filter characteristic of the IF amplifier and provide interference to the picture and/or sound of the desired program. The visual carrier of the desired channel is 45.75 MHz below the local oscillator frequency. The image signal will be 45.75 MHz above the local oscillator frequency or 91.5 MHz above the visual carrier of the desired channel, about 15 6-MHz channels away. The visual carrier is the strongest part of the interfering channel. Visual carriers are spaced 6 MHz apart in cable practice. The visual carrier 15 channels above the desired carrier will be 90 MHz higher in frequency. When local oscillator frequency is subtracted from it, the visual carrier in the image region will be 1.5 MHz above the desired visual carrier and produce an interfering beat in the picture. The aural carrier of the channel that is 14 channels above the desired channel will also fall into the video bandwidth of the desired channel.

Since the tuner determines the rejection at the image frequency, the level of rejection varies with the channel. All receivers did well on channel 3, and only four had problems with channel 12. Half the receivers had interference on channel 53, and 67% failed on channel 74.

24.4.8 Tuner Overload Performance

Tuner problems and other deficiencies may result in impaired performance on the TV or VCR itself. In addition to the DPU problem previously described, nonlinearity in the receiver’s circuits can give rise to the production of new frequencies not present in the original signal. The signals thus generated produce either diagonal bars in the picture or even a discernable image from another channel.

Tuner overload is a serious problem that will become more serious as cable bandwidth expands. Of the TVs, 66% had problems on at least one channel, 50% of VCRs suffered as well. Surprisingly, even 36% of the set-top terminals had troubles.

24.4.9 Rejection of Signals in the Upstream Band

Two-way services, once rare, are now commonly offered by cable operators. High-speed data service, two-way video set-top boxes, sometimes telephone interface units, and various types of status monitoring combined fill up a bandwidth whose upper limit has increased from 30 to about 42 MHz in recent years to meet the demand for additional upstream capacity. The result is that signals are generated at fairly high levels within customers’ homes and transmitted toward the headend. Due to the finite isolation offered by taps and in-house splitters, some of that energy is directed toward the input terminals of television receiving devices.

The problems are twofold. First, the amplitudes may be sufficient to cause input stages to distort, producing intermodulation products that may fall within the normally tuned channels. Second, the upper frequencies fall within the common television intermediate frequency band of 41-47 MHz and may leak directly into sensitive amplifier stages if not sufficiently rejected by the input circuits.

The magnitude of the reflected signals can be estimated as follows. The power level of transmitted signals from standard cable modems are controlled by the headend and must be adjustable to as high as +58 dBmV to meet the DOCSIS specification (though some models can go higher). For reasons discussed in Chapter 16, higher levels are consistent with reduced average levels of ingress, and cable operators design their plants so that as many modems as possible are transmitting near their maximum capabilities. Using the assumption in Section 24.4.2 that the sum of cable loss and tap or splitter isolation will be at least 20 dB, the maximum reflected power at the antenna terminals of a television receiver in the same or a neighboring house can be as high as +38 dBm.

The presence of signals of that magnitude falling below 30 MHz should not be a problem because television receivers are designed to reject signals below that frequency with magnitudes as high as — 7 dBm (+ 42 dBmV). At the other extreme, some receivers offer little protection for signals falling at or above 41 MHz, with levels as low as +10 dBmV causing visible interference in some models. IS-23 is silent on the issue of receiver protection above 30 MHz and also silent regarding maximum levels in that range that may be delivered by cable operators. Clearly, however, without some form of additional protection or avoidance of at least the frequencies above 41 MHz, many receivers will generate unacceptable levels of picture degradation when nearby two-way devices are transmitting near the upper end of the upstream band. The testing described earlier did not include this critical parameter.