Fundamentals - The MPEG - page 31

When speed is reduced, the sampling rate falls, and a fixed filter will allow part of the lower sideband of the
sampling frequency to pass. If the sampling rate of the machine is raised, but the filter characteristic remains the
same, the problem can be avoided, as in (c).
In the early days of digital audio, video recorders were adapted to store audio samples by creating a pseudo-video
waveform which could convey binary as black and white levels. [ 9 ] The sampling rate of such a system is
constrained to relate simply to the field rate and field structure of the television standard used, so that an integer
number of samples can be stored on each usable TV line in the field. Such a recording can be made on a
monochrome recorder, and these recordings are made in two standards, 525 lines at 60 Hz and 625 lines at 50 Hz.
Thus it was necessary to find a frequency which is a common multiple of the two and also suitable for use as a
sampling rate.
The allowable sampling rates in a pseudo-video system can be deduced by multiplying the field rate by the number
of active lines in a field (blanked lines cannot be used) and again by the number of samples in a line. By careful
choice of parameters it is possible to use either 525/60 or 625/50 video with a sampling rate of 44.1 kHz.
In 60 Hz video, there are 35 blanked lines, leaving 490 lines per frame, or 245 lines per field for samples. If three
samples are stored per line, the sampling rate becomes
60 x 245 x 3 = 44.1 kHz
In 50 Hz video, there are 37 lines of blanking, leaving 588 active lines per frame, or 294 per field, so the same
sampling rate is given by
50 x 294 x 3 = 44.1 kHz
The sampling rate of 44.1 kHz came to be that of the Compact Disc. Even though CD has no video circuitry, the
equipment used to make CD masters was originally video-based and determined the sampling rate.
For landlines to FM stereo broadcast transmitters having a 15 kHz audio bandwidth, the sampling rate of 32 kHz is
more than adequate, and has been in use for some time in the United Kingdom and Japan. This frequency is also
used in the NICAM 728 stereo TV sound system, in DVB audio and in DAB. The professional sampling rate of 48
kHz was proposed as having a simple relationship to 32 kHz, being far enough above 40 kHz for variable-speed
operation, and having a simple relationship with 50 Hz frame rate video which would allow digital video recorders to
store the convenient number of 960 audio samples per video field. This is the sampling rate used by all production
DVTRs. The field rate offset of 59.94 Hz video does not easily relate to any of the above sampling rates, and
requires special handling which is outside the scope of this topic. [ 10 ]
Although in a perfect world the adoption of a single sampling rate might have had virtues, for practical and
economic reasons digital audio now has essentially three rates to support: 32 kHz for broadcast, 44.1 kHz for CD,
and 48 kHz for professional use. [ 11 ] In MPEG these audio sampling rates may be halved for low bit rate applications
with a corresponding loss of audio bandwidth.
[ 9 ] Ishida, Y. et al. , A PCM digital audio processor for home use VTRs. Presented at 64th AES Convention (New
York, 1979), Preprint 1528
[ 10 ] Rumsey, F.J. and Watkinson, J.R., The Digital Interface Handbook . Oxford: Focal Press (1995)
[ 11 ] Anon., AES recommended practice for professional digital audio applications employing pulse code modulation:
preferred sampling frequencies. AES5-1984 (ANSIS4.28-1984), J. Audio Eng. Soc. , 32 , 781-785 (1984)
2.9 Video sampling structures
Component or colour difference signals are used primarily for post- production work where quality and flexibility are
paramount. In colour difference working, the important requirement is for image manipulation in the digital domain.
This is facilitated by a sampling rate which is a multiple of line rate because then there is a whole number of
samples in a line and samples are always in the same position along the line and can form neat columns. A
practical difficulty is that the line period of the 525 and 625 systems is slightly different. The problem was overcome
by the use of a sampling clock which is an integer multiple of both line rates.
ITU- 601 (formerly CCIR-601) recommends the use of certain sampling rates which are based on integer multiples
of the carefully chosen fundamental frequency of 3.375 MHz. This frequency is normalized to 1 in the document.