User manual

Audio Generator
AM700 Audio Measurement Set User Manual
4-25
Word Size. Use this field editor to specify the number of audio bits that are sent in a
digital signal word (the maximum word size that can be sent on an AES interface is
24 bits). Change the word size by touching the Word Size field editor and entering a
new value at the keypad. Press the Enter key to accept the entry. After touching the
Word Size field editor you can also turn the large knob to display a new value.
Touch the field editor icon (or press the Enter key) to accept the word size value.
The ability to change the word size can be used, for example, in testing the behavior
of a receiver while changing the number of audio bits it receives. A receiver can
either truncate or dither the input when receiving a word size larger than it is
designed to accept.
Dither Type. Selections are None, Rectangular PDF, or Triangular PDF. An 18-bit
signal is typically produced by generating a 24-bit signal and either truncating or
rounding it. When generating signals of fewer than 24 bits, dithering reduces
truncation noise. The dither selections allow you to choose the type of probability
density functions (PDF) used in dithering.
Channel Status Mode. Touching this soft key displays a menu from which you can
select the type of signal produced by the audio generator. Selections are Profes-
sional, Consumer, Non Audio, and File.
Every audio sample has a specific number of audio bits. Included with the audio bits
are validity, user, channel status, and parity (VUCP) bits. Channel status (C)
information is organized in 192-bit blocks that are subdivided into 24 bytes.
Examples of information carried in channel status are: length of audio sample
words, number of audio channels, sampling frequency, sample address code,
alphanumeric source and destination codes, and emphasis.
The Channel Status mode soft key allows you to specify the type of audio signal
generated by the AM700. Channel status information sent by the audio generator
can cause specific behavior or display messages on equipment receiving the test
signal.
You can also select a file (created on a PC, saved to a floppy disk, and transferred to
the AM700) from which the audio generator reads channel status information. Refer
to AES3-1992 (ANSI S4.40-1992) for more information about channel status.