Audio compression - The MPEG - page 106

Figure 4.7: Some of the scales used in conjunction with the PPM dynamics. (After Francis Rumsey, with
permission.)
However, in the situation where a digital copy of an analog tape is to be made, it is very easy to set the input gain
of the digital recorder so that line-up tone from the analog tape reads 0 dB. This lines up digital clipping with the
analog operating level. When the tape is dubbed, all signals in the headroom suffer convertor clipping.
In order to prevent such problems, manufacturers and broadcasters have introduced artificial headroom on digital
level meters, simply by calibrating the scale and changing the analog input sensitivity so that 0 dB analog is some
way below clipping. Unfortunately there has been little agreement on how much artificial headroom should be
provided, and machines which have it are seldom labelled with the amount. There is an argument which suggests
that the amount of headroom should be a function of the sample wordlength, but this causes difficulties when
transferring from one wordlength to another. In sixteen-bit working, 12 dB of headroom is a useful figure, but now
that eighteen- and twenty-bit convertors are available, 18 dB may be more appropriate.
4.4 The ear
The human auditory system, the sense called hearing, is based on two obvious tranducers at the side of the head,
and a number of less obvious mental processes which give us an impression of the world around us based on
disturbances to the equilibrium of the air which we call sound. It is only possible briefly to introduce the subject
here. The interested reader is referred to Moore [ 3 ] for an excellent treatment.
The HAS can tell us, without aid from any other senses, where a sound source is, how big it is, whether we are in
an enclosed space and how big that is. If the sound source is musical, we can further establish information such as
pitch and timbre, attack, sustain and decay. In order to do this, the auditory system must work in the time,
frequency and space domains. A sound reproduction system which is inadequate in one of these domains will be
unrealistic however well the other two are satisfied. Chapter 3 introduced the concept of uncertainty between the
time and frequency domains and the ear cannot analyse both at once. The HAS circumvents this by changing its
characteristics dynamically so that it can concentrate on one domain or the other.
The acuity of the HAS is astonishing. It can detect tiny amounts of distortion, and will accept an enormous dynamic
range over a wide number of octaves. If the ear detects a different degree of impairment between two audio
systems and an original or 'live' sound in properly conducted tests, we can say that one of them is superior. Thus
quality is completely subjective and can only be checked by listening tests. However, any characteristic of a signal
which can be heard can in principle also be measured by a suitable instrument, although in general the availability
of such instruments lags the requirement and the use of such instruments lags the availability. The subjective tests
will tell us how sensitive the instrument should be. Then the objective readings from the instrument give an
indication of how acceptable a signal is in respect of that characteristic.
Figure 4.8 shows that the structure of the ear is traditionally divided into the outer, middle and inner ears. The outer
ear works at low impedance, the inner ear works at high impedance, and the middle ear is an impedance-matching
device. The visible part of the outer ear is called the pinna which plays a subtle role in determining the direction of
arrival of sound at high frequencies. It is too small to have any effect at low frequencies. Incident sound enters the
auditory canal or meatus. The pipe-like meatus causes a small resonance at around 4 kHz. Sound vibrates the
eardrum or tympanic membrane which seals the outer ear from the middle ear. The inner ear or cochlea works by