Information Technology Reference
In-Depth Information
that audio compression is not worth while. Only when high video compression factors are used does it become
necessary to apply compression to the audio.
Professional equipment traditionally has operated with higher sound quality than consumer equipment in order to
allow for some inevitable quality loss in production, and so there will be less pressure to use data reduction. In fact
there is even pressure against the use of audio compression in critical applications because of the loss of quality,
particularly in stereo and surround sound.
With the rising density and falling cost of digital recording media, the use of compression in professional audio
storage applications is hard to justify. However, where there is a practical or economic restriction on channel
bandwidth compression becomes essential. Now that the technology exists to broadcast television pictures using
compressed digital coding, it is obvious that the associated audio should be conveyed in the same way.
4.14 Audio compression tools
There are many different techniques available for audio compression, each having advantages and disadvantages.
Real compressors will combine several techniques or tools in various ways to achieve different combinations of
cost and complexity. Here it is intended to examine the tools separately before seeing how they are used in actual
compression systems.
The simplest coding tool is companding which is a digital parallel of the noise reducers used in analog tape
recording.
Figure 4.22
(a) shows that in companding the input signal level is monitored. Whenever the input level
falls below maximum, it is amplified at the coder. The gain which was applied at the coder is added to the data
stream so that the decoder can apply an equal attenuation. The advantage of companding is that the signal is kept
as far away from the noise floor as possible. In analog noise reduction this is used to maximize the SNR of a tape
recorder, whereas in digital compression it is used to keep the signal level as far as possible above the distortion
introduced by various coding steps.
Figure 4.22:
Digital companding. In (a) the encoder amplifies the input to maximum level and the decoder
attenuates by the same amount. (b) In a companded system, the signal is kept as far as possible above the noise
caused by shortening the sample wordlength (c).
One common way of obtaining coding gain is to shorten the wordlength of samples so that fewer bits need to be
transmitted.
Figure 4.22
(b) shows that when this is done, the distortion will rise by 6 dB for every bit removed. This
is because removing a bit halves the number of quantizing intervals which then must be twice as large, doubling
the error amplitude.
Clearly if this step follows the compander of (a), the audibility of the distortion will be minimized. As an alternative to
shortening the wordlength, the uniform quantized PCM signal can be converted to a non-uniform format. In non-
uniform coding, shown at (c), the size of the quantizing step rises with the magnitude of the sample so that the
distortion level is greater when higher levels exist.
Companding is a relative of floating-point coding shown in
Figure 4.23
where the sample value is expressed as a
mantissa and a binary exponent which determines how the mantissa needs to be shifted to have its correct
absolute value on a PCM scale. The exponent is the equivalent of the gain setting or scale factor of a compandor.
