Matters Concerning Audio - A Practical Guide to Video and Audio Compression - page 153

Two track tape recorded with a mono track each side.

A

B

Both tracks sampled in via a stereo input but one is in reverse direction.

A

B

A B Reversed

Figure 7-5 Multi-track audio swapping in sound editor.

7.4 How the Audio Is Coded and Synchronized

Audio is an analog waveform. The human ear responds to it as a change in air pressure
and each ear receives a 1-dimensional signal. When we listen to sounds, we place them
fairly accurately into a 3D space but this is still happening via just two 1-dimensional sig-
nals, one for each ear.
The relative phasing and arrival times of those waveforms are what makes sounds
appear to move forward, backward, and up and down. The separation of the sounds and
the panning is what moves them left to right. This is a very simplified description of what
happens and if you need to know more, please consult some topics on psychoacoustics.
Certain sound tracks are recorded with special attention to the placement of the micro-
phones. This extends to using a dummy head to hold the microphones and placing them
where the human ears would be. Even the shape of the ears is modeled to accurately pick up
exactly what a human being would hear, although this can also be done by carefully selecting
and positioning microphones with an appropriate frequency response and dynamic profile.
At playback time, the optimum experience is obtained by listening on a pair of head-
phones. This is called binaural recording and everything about the quality of the audio
depends on the accuracy of the recording levels and the phasing of the channels. Lowering
the sample rate or compressing the audio too much will destroy the subtleties of this
recording and it will be impossible to reconstruct the missing information on playback.

7.4.1 Sample Rates and Nyquist (44.1, 48, 96, and 192 KHz)

Converting from an analog source to a digital storage medium requires an instantaneous
measurement of the sound level and a notation of the value. These measurements are called