NICAM Digital Stereo Sound

adequate.As is only to be expectedwith such a reasonably small-valuecapacitor of course. priced unit, there's no bar graph. I feel that Cirkit has come up with a range of real The meter performed beautifully on test. It was very winners with thesemeters. For the price the TM175 offers easyto use consideringthe number of ranges.To start with exceptional quality and features. You can't expect the we were rather dubious about the frequency counter, but build quality of more expensive machines such as those when it was checkedagainst our Racal unit we found that it produced by Fluke, but the test unit gave no cause for had a faster responseand a perfectly acceptableaccuracy. concern on this score. I wouldn't hesitate to recommend Having a capacitor tester is a real boon. Previouslythe best the unit to anyone in the service industry. My thanks to we could do was to carry out a resistancecheck to see a Cirkit Distribution, Park Lane. Broxbourne. Herts EN10 capacitor charge. This doesn't tell you much with a 7NQ for providing the test unit. NICAM Digital StereoSound Part 1 Eugene Trundle For many years sound was the "poor relation" in what goes into the microphone and comes out of the television. Most TV sets had a tiny, often side-mounted loudspeaker is a sound pressure wave. Conversion to loudspeaker which was driven by a barely adequate digital form, by quantisation (sampling the audio signal at amplifier. The situation remains much the samewith many regular intervals and assigning to each sample a digital TV designs. But the advent of hi-fi VCRs and the value), is done to overcome deficienciesin the intervening integration of audio and video equipment has in recent systemsand equipment. When reproduced, the quality of years led to a great deal more attention being paid to digital sound, and particularly its signal-to-noise ratio, "with-picture sound". The f.m. TV sound system that largely dependson the number of quantisationlevels used. came into use with 625-line transmissionshas served us The ultimate, as used in the CD and R-DAT systems,is 16 well, providingquality soundto which receivingequipment bits. giving over 65,(X)0values each representing a signal seldom does justice. But it's incapable of carrying stereo. amplitude level. A 16-bit signal would require a wideband and various other shortcomings become noticeable when channel for transmissionhowever. As there is not room to it's processedby high-quality receivers, recorders. ampli- accommodatesuch a signal in the broadcastTV bands the fiers and loudspeakers. Nicam system uses l0-bit words. Ordinarily, these would Stereo TV sound has been available in W. Germany and provide a signal-to-noseratio of about 60dB. An improve- Japan for some years. The systemsused in these countries ment is achievedby continuously "moving the baseline" as employ analogue modulation of an audio subcarrier or a it were. This gets close to the 84dB signal-to-noiseratio second carrier by a stereo-differencesignal - the systems provided by a 14-bit system. Just how this is done will be are in many ways similar to the technique used for explainedlater. v.h.f.-f.m. radio broadcasting.Encoding and modulation The 32kHz sampling rate used by the Nicam system systemsof this type don't fit easily alongsidea video signal provides a frequency responseof 0-15kHz in two channels in the tightly packed broadcast bands however, and many for stereo. Separationbetween the two channelsis total, so compromlsesare necessary. that dual-languagesound tracks can be broadcastwithout Research and development by the BBC led to the crosstalk.The Nicam systemcan carry data in one or both publication in 1986 of a specification for a radically channelsand is completely separatefrom and independent different TV sound system for broadcasting. It takes full of the existing f.m. sound channel at 6MHz (system I). advantage of modern digital technology and exploits the Before it was adopted the Nicam system had to be tested ability of chip manufacturers to mass-producevery com- under many different sets of conditions. It proved to be plex digital i.c.s at low unit costs.ln short, it representsthe extremely rugged, providing high-fidelity stereo sound current "state of the art". Setmaker Ferguson Ltd. did a even when the signal is so weak that the picture can hardly great deal of decoder designwork, which culminated in the be locked. Texas Instruments' single-chip demultiplexer that's cur- rently used in many TV and VCR Nicam designs. The System Overuiew system is officially known as NICAM 728 - Near Instan- taneously Companded Audio Multiplex, the 128 referring Many of the processesand techniques that the Nicam to the data rate. system uses are common to those employed by the compact disc system. Thus readers who are familiar with the CD system - primed, perhaps, by Joe Cicszynski's Digital Sound excellent seriesof articles in this magazine (March 1989- July 1990) - will have little trouble in understanding and Modern sound systemsare based on the use of digital servicingNicam equipment. Fig. 1 provides an overall view technologv. The first example that came along in the of the system,which we'll describein outline before going domesticelectronics field was the compact disc (CD) which into detail. was introduced by Philips and Sony in 1982. Pulse-code The studio sound feeds for the left and right channels are modulated (PCM) sound was first introduced in the video first pre-emphasisedand passedto an analogue-to-digital field with the Video-S format. We will soon have more converter which produces t4-bit audio samples("words") record and playback digital audio technology in the forms at a sampling rate of 32kHz. Thcse samplesare then t'ed to a of VHS-PCM and R-DAT. compressor which reduces the bit rate to 10 per word, Audio signalsmust start and end in analogueform, i.e. effectively extracting the ten most useful bits and discard- 846 TELEVISIONSEPTEMBER 1990 ---- I S l l r I ) ) Tone Bal ance Stereo wide Fig. 1: Block diagram of the Nicam system, (a) at the studio/transmitter end, (b) at the receiver end. ing the rest. At this stage an error bit is added for error It remains to convert the data back to analogue form, checking and correction in the receiver. In the following using a DA convertor, then to filter the result to obtain the interleaver stage the bits are shuffled like a pack of cards. original sound signals.User controls are incorporated into This is done to distribute the effects of propagation errors the audio amplifiers, whose design is critical if the low or interference when the original order is restored at the distortion and high signal-to-noiseratio of the transmitted receiving end. Scrambling is then done by adding to the signal are to be maintained. Thc performance, rating and interleaved data an effectively random data stream, so that positioning of the loudspeakers are also crucial to the the sidebands of the transmitted Nicam signal have no realisation of the full potential of Nicam sound. strong, fixed-frequency components that could cause interference. Modulation comes next. For system I the Nicam An lncongruity subcarrier is at 6.552MH2. It's phase modulated by the data pulses, using the DQPSK system that we'll describe The incongruity with many types of programme of later. With digital signalsphase modulation is economical havingthis wide, high and full-bodiedsound alongside the of bandwidth, a factor that's a key to being able to squeeze necessarilylimited picture area is somethingwe won't yet another signal into the tightly-packed 8MHz slot dwell upon here. The systemcomes into its own with available for a system I TV channel. The Nicam carrier is concertsof any type, and with any Nicamprogramme the added to the 6MFIz f.m. sound carrier fed to the larger the screenthe better. In one jump the TV sound transmitter, where it joins the vision carrier (at a level of systemhas got itself severalstrides ahead of the picture -20dB relative to the peak vision carrier level) on its way transmissionand displaycapabilities. to the transmitting aerial. At the receiver the tuner converts the Nicam carrier Sysfem in Detail to an i.f. of 32.95MH2. This is demodulated and selected by a filter for application to the first section of the Nicam Although it's not essentialfrom the servicingpoint of decoder, the DQPSK demodulator. The Nicam signal view to understandwhat goes on in the studio and the emerges from this as a data stream. It then undergoes a encodingand even the decodingprocesses, a reasonable series of processes that reverse those carried out at the knowledgeof how the systemworks doeshelp with fault broadcasting end. First comes a descrambler which sub- diagnosisand Nicam decodersetting up. Nicam decoders tracts from the data stream the random data added at the are now becominga common feature in top-of-the-range transmitter to smooth out the sidebands.De-interleaving, TV setsand VCRs. in accordance with a simple codebook held in memory. then takes place. As a result the order of the original 10 + Pre-emphasis 1 bit words is restored. The words are next expanded to 14-bit form in accordance with a scale factor transmitted First, pre-emphasis.No soundor visiontransmission or with the data. Error correction on the basis of the parity recording system is complete without a pre-emphasis bits is then undertaken. At the end of this decodingprocess system,which givesa boost to the high-frequencycompo- we have a real-time data stream of 14-bitwords, containing nents of the signal. During reproduction the balance is information for both the left and risht sound channels.

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