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craig@yonder.equinox.gen.nz (Craig Shore) writes:

>Has anyone got, or does anyone know of a place that has, information on
> NICAM stereo.  Someone told me today that it's an inferior system
> to other stereo broadcasts used in the world and I want to find out
> if that's true or not. NICAM is a digital system isn't it? 
>Thanks for any help,
>    Craig.

---------------------------------------------------------------------------------
Stereo in TV?                   1992-07-17

A couple of years ago stereo TV sound was introduced here in Sweden, and in the
winter of -92, when my old VCR needed replacement, I decided to get a HiFi-VCR
with NICAM decoder. The difference in perceived sound quality was enormous
(mostly because I now have STEREO!) so I tried to find out what NICAM really
is and how it works.

TV transmissions with stereo sound is nothing new, analog stereo is used in
many countries: in Japan since 1978, in (West)Germany since 1981 and in US
since 1984, I think.

NICAM (Near Instantaneous Companded Audio Multiplex) 728 is a European system
developed by BBC and a British company called IBA, that developed the
satellite standard MAC. It is adapted from MACs sound standard so that the
same circuits/coding can be used both for terrestial- and satellite-TV reception.

NICAM 728 was developed to meet several reqirements: 
 To be compatible with the old system, no interference.
 To increase the sound quality significantly, not only by adding stereo.
 The possibility to transmit two seperat sound channels, bilingual trans-
 missions.
 The possibility to transmit data instead of sound.
 The possibility to encrypt the transmission of sound.
 The new sound should be resistent to 'normal' interference, ie like the FM
  sound: a little better then the picture signal. (I can pick up NICAM even
  when the picture has degraded so much that I don't get colour and the FM
  sound is very noisy. One thing to note is that the loudest noises distort
  when you pick up a marginal signal.)

Nicam is a digital system with 32kHz sampling frequency and 14bit resolution.
The data is 'bundled' in frames, each frame is 1 ms of(stereo)sound.

Each frame consists of: a frame sync word of 8 bits, 1 flagbit, 4 control bits,
11 'additional data' bits and 64 samples of 11 bits(10 data + 1 parity bit)
'sound'. This gives a bit rate of 728 kbit/s. Hence the full name: NICAM 728.

The bit rate is reduced by only transmitting 10 bits of each sample. The amp-
litude of the 'loudest' sample in each frame decides wich 10 bits that should
be transmitted, because of this there is a 'scale factor' transmitted with each
frame. To save even more 'bits' the parity bits in the sound samples is used as
'scalebits'. The resolution in 'high level' frames is reduced and you get a
sort of 'program modulated' noise. The noise is partly masked by the (relative)
high level of the signal and partly by a pre-emphasis of the high frequencies
(I don't have any figures). After expansion there is of course a de-emphsis,
as in ordinary FM tuners.

The content of each frame is defined by the 4 control bits. 3 of the bits are
used to set different operating conditions: stereo, 2 mono sounds, 1 mono sound
and one 352 kbit/s data channel or one 704 kbit/s data channel. The 4th control
bit is used to indicate if the analog sound is the same as the NICAM sound
giving receivers the possibility to switch over to analog in case of
disturbances.

The use of the 11 bits of 'additional data' is not yet defined. (It is supposed
 to be able to carry teletext information too, although this is not yet used as
 far as I know.)

Before transmission the bits are interleaved by 16 bit positions to reduce the
influence of short dropouts. Also the whole frame, except the frame sync, is
'scrambled' to get a more random energy spectra and further reduce the influence
of disturbances.

The NICAM carrier is DQPSK(Differently encoded Quadrature Phase Shift Keying)
modulated at a level 20 dB lower then the picture carrier, 7 dB lower then the
analog sound.
In PAL B, what is what we have got here in Sweden, the (mono)sound is FM modu-
lated 5,5 MHz above the picture carrier. The NICAM carrier is modulated at
5,85 MHz above the picture carrier.

In april 1987 EBU (European Broadcasting Union) recommended NIACAM for PAL B,
G and I. That is the systems used in most European countries.

The NICAM system limits the transmitted sounds frequency response to 15kHz
and the dynamic range to 85dB (I really doubt the last figure) and a channel
separation of 85dB. 


Conclusion
If you live in a PAL country (except Germany) and want stereo sound when
watching TV, you need NICAM. But also if your (TV mono)audio reception is
disturbed you can get much better (mono)audio with NICAM. The perceived
audio difference, at optimum reception, between NICAM and FM-mono is a
question of opinion and quality of the hardware involved(both in the trans-
mitting and receiving end).

Some NICAM equipped 'netters' claim that for them the only advantage with NICAM
over FM-mono is a better signal to noise ratio, but as I said in the beginning
the difference in perceived sound quality when I switch between NICAM and FM-
mono is great. I can even hear a quality difference on mono sources :-).

Personally I think that NICAM was developed NOT to bring HiFi-Audio quality
sound to TV viewers. But rather it was developed for higher channel separation
and good resistence against interference instead of bandwidth or 'resolution'.
High channel separation is the key to good bilingual reception.

Since NICAM is a European system, the first TV/VCRs with decoders was European
and when the Japanese equipment was introduced it didn't take long before there
was a lot of rumors about the Japanese decoders not working properly, even as
newly as april -92 I read that the Japaneese decoders introduces a delay between
the decoded channels so you get less then optimum performance if you use your
NICAM decoder for Dolby Surround.

So if you don't hear any quality difference between NICAM and the old analog
TV-sound it MAY be because your equipment is built with a Japanese NICAM decoder.
I haven't heard any Japanese NICAM decoders so I don't know. My VCR is a Philips.
But there is a long way from the TV-studio/tape machine to the viewer in his/her
living room and even more ways to mess up the sound!

BTW (non Japanese)NICAM is supposed to work very well with Dolby Surround. I
don't know I have not tried it, but I certanly can here the 'phase effects',
watching a Dolby Stereo encoded movie.


This is what I have found about NICAM, there are probably both errors and a
lot of missing info. Please feel free to correct(add) anything that you feel
is wrong(missing).
---------------------------------------------------------------------------------
Hakan Carlson
ELLEMTEL Telecommunication Systems Laboratories
SWEDEN