WHY BOTHER WITH STEREO?
WHY BOTHER WITH STEREO?
By Frank McClatchie:
FM SYSTEMS, INC.
Basically – Because your subscribers expect it! They are so used to their music and movies being in stereo, that if their TV programming is NOT in stereo – something seems missing. With more and more people investing “big money” into sophisticated Surround Sound systems, the demand for better quality stereo is only going to increase. Stereo creates a sense of involvement and intimacy for the audience, something that both programmers and advertisers strive for.
Much has been written about the technical aspects of BTSC stereo. You will find a few sources mentioned in the handout. Today I will give you an overview of the way BTSC stereo works, with particular attention paid to how to test your Stereo Performance and discuss one of the new technologies for stereo transmission.
So, how is BTSC different from good old FM stereo? Not very much in the basic processes, but those differences create dramatic problems at the transmitters, Cable head-end, Set-top Converter, and mono TV sets.
First of all, let’s discuss the similarities that you can see in figure 1 of the handout. Note that at first glance, the only differences seems to be shifting of the pilot L-R carrier frequencies to accommodate the high level horizontal line rate harmonics that would otherwise induce excessive “Whistles” in the received stereo. So the basic process is the same. It is the difference between FM stereo and the BTSC systems that cause most of the problems.
The major difference is that the L-R audio channel is companded. Companding is necessary because TV audio is only deviated +/- 25 KHz, while FM stereo has the FM advantage of +/- 75 KHz deviation. Since BTSC must be compatible with existing Monaural TV sets, the L+R deviation must be +/- 25 KHz. You can see this illustrated in figure 2.
The first step in overcoming excess noise in stereo was to double L-R deviation to +/- 50 KHz. The next step was to compress the Dynamic Range of the L-R signal at the transmitting end, then expanding back to the original dynamic range at the stereo TV receiver.
All of this is necessary to produce a good signal to noise ratio. This extra complication places a heavy emphasis on maintaining absolute level and phase continuity between the BTSC multiplexer at the transmit end and the de-multiplexer in the TV receiver.
We are talking about +/- 0.05 dB and 0.1 degree accuracy for multiplexers, with overall limits for all segments of the transmission system taken together to be no more than +/- 3 degrees and +/- 0.4 dB to obtain 30 dB separation. You can see this on the chart labeled figure 3.
The DBX system used in the BTSC system compands the full audio spectrum by a 2:1 ratio, then adds spectral compression, which in effect compands the higher frequencies by 3:1, multiplying any high frequency transmission errors by a factor of three. See figure 5. This is a tall order to be met every day in the real world. The consequence is that most TV stereo tends to be closer to 10-12 dB separation at the TV set. Each part in the transmission system can add to this degradation.
In a cable system receiving BTSC from a TV station, the main problem once the TV transmitter is correctly aligned, tends to be at the cable head-end. Two main types of off-air converters are often in use. The base-band conversion type typically blocks BTSC altogether, although newer designs can pass the BSTC signal. The RF conversion variety can transmit BTSC to varying degrees of fidelity, but in this case, it is the sound carrier trap that induces phase shift of varying severity, as shown in figure 4.
The sound carrier amplitude must be reduced to 15 dB below video carrier amplitude. Monaural sound is not affected, but stereo separation can be reduced to nil, even by RF conversion. One solution is to transmit stereo by land line or microwave to the cable head-end and encode it there.
The set-top converter can seriously affect BTSC, too. Some of the older Z-Tac and Tocom converters can completely kill BTSC stereo, although the newer versions of these base-band converters can pass BTSC stereo reasonable well. In general, RF Set-top converters do not effect BTSC stereo separation.
Now let’s discuss satellite and microwave delivery of stereo. Most satellite stereo is sent on separate left and right channels, either within the video signal (such as B-Mac or Videocipher), or on two extra Sub-carriers. The stereo arrives at a Cable head-end as left and right channels that must encoded into BTSC and modulated onto the 4.5 MHz sound carrier.
A Cable system may have 10-20 BTSC multiplex modulators instead of only one as in a broadcast TV station, so the Cable Operator has all the same problems with AM-PM and PM-AM conversions, as well as ICPM (Incidental Carrier Phase Modulation) and crosstalk that the broadcaster has, except that he has 10-20 times as many to care for.
Some cable systems receive TV channels through Frequency Modulated links or FMLs, or via Amplitude Modulated Links or AMLs, from some other signal receiving locations. The AML will pass BTSC essentially un-impaired, except for some decrease in signal-to-noise ratio. The FML will usually transmit the sound channel at 4.5 MHz, and will pass BTSC with some impairment of stereo separation, depending mostly on the Phase characteristics of the video low pass filter used in the system. Other FML systems transmit TV audio at 5.8 MHz or other Sub-carrier frequencies. In this case special arrangements must be made at both ends of the Microwave to pass BTSC signals.
So how good is good enough? And what is stereo anyway? After the real world difficulties of transmitting stereo by BTSC became manifest, a number of studies were done that purport to show that not much stereo separation is needed anyhow. Some studies have shown that listeners could localize sound sources with as low as 7 dB separation, however not many of us would consider that good stereo!
On the other hand, few people would notice a significant improvement in stereo of the stereo separation was increased from 25 dB to say 40 dB. The minimum target for overall system should be about 20 dB. As we have seen, even 20 dB is a lofty goal for the overall system.
All stereo transmission systems are an attempt to Synthesize the original acoustic environment. The more channels (each carrying different versions of the original), the better the illusion can be. Witness the various “quad” systems that have been tried. Two channels generally create 80% of the illusion, while adding more only close the gap slightly, while increasing the cost of the transmission system greatly.
There is nothing magic about “Left and Right”. In fact, in most of today’s recordings, there is no Left and Right microphone. Most likely there are about 16 microphones and the person that mixes all these sources together does so to produce a pleasing effect to his ears. In practical fact, he is producing two different sound channels, that when reproduced by two sets of speakers, will sound pleasing to the ears.
There is usually no relationship between Left and Right and the sound coming down the cannels so designated. The main important ingredient is that the two channels contain different waveforms. If the waveforms are identical, the sound will be perceived as monaural, even though the sound emanates from two speakers. If the waveforms are some what coordinated, and yet different in detail, greater stereo separation will be perceived. All of the preceding discussion leads to some interesting conclusions.
First, that identifiable left and right channels are not necessary to create the perception of stereo. Second, that the perception of stereo is related to the degree of Non-identity of the two audio channels. And third, that there may be ways of transmitting two non-identical channels that would be less sensitive than BTSC to Amplitude and Phase shifts in the transmission system, and yet would decode on the BTSC de-multiplexing system in the stereo TV set.
Such MTS multiplexers can be decoded very well in a BTSC receiver and yet, can deliver a very strong perception of stereo under conditions of amplitude and phase shift in the transmission system sufficient to all but destroy stereo separation in a signal originating from a BTSC multiplexer.
The FMT633S is a TV stereo synthesis multiplex modulator that will create a stereo synthesis signal from a monaural base-band audio source. Its output is 4.5 MHz to match the cable TV sub-carrier modulator input. The audio synthesis and companding system work vary well with all stereo TV receivers.
