Audio Delay Buffer
The audio delay buffer stores the converted samples in a circular buffer. The samples
are read out the appropriate time later. The audio delay can be specified in two parts:
fixed and variable. The variable delay is used for the Auto-Track feature, and is
implemented through control of the sample rate converter. The operator specifies the
fixed delay as a number of milliseconds of delay. With 521Kx16 memory, and three
words of memory for each sample of a channel pair, and two channel pairs, the
maximum fixed delay is 87381 samples. At a sample rate of 48kHz, this translates
into 1.8 seconds of delay. The variable delay may be disabled under control of the
operator.
The audio delay buffer is controlled through the FPGA.
Auxiliary Audio Stream
The auxiliary audio stream allows for insertion of an arbitrary user-specified audio
stream into the processed channel. The stream is downloaded into the auxiliary audio
stream buffer, using a bank-switched buffer allocation scheme. Software support
allows for downloading the auxiliary stream from an external computer, and then
playing it back through the audio port. The stream may be of any arbitrary length,
subject to memory restrictions on the main processor. The stream may be processed
as one stereo pair of channels, or as two mono channels.
Programmable Tone Generator
The programmable tone generator allows for four digital audio tones to be generated.
Each tone has a buffer size of 8K samples. The number of samples in the buffer will
determine the accuracy of the desired tone frequency. For standard audio tones of
1KHz, 2KHz 4KHz, 10KHz, and 440Hz, 8K samples are sufficient.
Multiplexor
The output multipleorx allows for insertion of the test tones or the auxiliary data
stream on to the regular channel streams. The multiplexor also allows for
manipulation of the Left and Right signals of the stereo pair. Possible actions are
mute L or R, swap L or R, Mono L or R, Mono-Sum (L and R are set to (L+R)/2),
tone substitution L/R, Auxiliary stream substitution L/R, or Auxiliary stream mixing
(L is set to AL+L)/2).
Digital Gain
The digital gain allows for adjusting the audio levels in the digital domain. The
adjustment can be up to +/- 20dB. This control is implemented inside the FPGA.
Each channel of the stereo pair has a different gain value applied to it, allowing for
balance adjustments. The output of the digital level control is two I
2
S streams that are
locked together.
The actual level of the audio sample is monitored. The maximum value during the
monitoring period is stored as an 8-bit number. Separate numbers are stored for the L
and R channels. The microprocessor reads the numbers and clears them on a periodic
basis.
Independent digital gain and level blocks are applied to the regular processed data
streams (2), the SDI Digi-Plex streams (2), and the auxiliary audio stream (1).
Digital Audio Transmitter
The digital audio transmitter accepts the I
2
S serial data stream and generates the
AES/EBU serial data stream. The transmitter supports both 110Ω and 75Ω formats.
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DPS-475/575 Service Manual
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