GS2970 Datasheet, PDF (85/145 Pages) Semtech Corporation – 3Gb/s, HD, SD SDI Receiver Complete with SMPTE Audio and Video Processing

4.19.3.3 Audio FIFO Block
The function of the FIFO block is to change the audio data word rate from the ANC rate
multiplexed with the video signal to the 48kHz audio output rate.
The audio FIFO block contains the audio sample buffers; one per audio channel. Each
buffer is 36 audio samples deep. At power up or reset, the read pointer is held at the zero
position until 26 samples have been written into the FIFO (allows for 6 lines per frame
with no audio samples; a maximum of 4 samples per line in SD Mode). See Figure 4-43.
Read
Pointer
Address 0
25
35
Audio Buffer
Write
Pointer
Figure 4-43: Audio Buffer After Initial 26 Sample Write
The position of the write pointer with respect to the read pointer is monitored
continuously. If the write pointer is less than 6 samples ahead of the read pointer (point
A in Figure 4-44), a sample is repeated from the read-side of the FIFO. If the write pointer
is less than 6 samples behind the read pointer (point B in Figure 4-44), a sample is
dropped. This avoids buffer underflow/overflow conditions.
Read
Pointer
Address 0
5
29
35
Audio Buffer
A
B
Write
Pointer
Figure 4-44: Audio Buffer Pointer Boundary Checking
Write
Pointer
The repeat or drop sample operation is performed a maximum of 28 consecutive times,
after which the audio outputs are muted (all sample data set to zero). In SD Mode, 26
samples are required to be written into the FIFO prior to starting the read operation
again.
The audio buffer pointer offset may be reduced from 26 samples to 12 or 6 samples using
the OS_SEL[1:0] bits in the host interface. The default setting is 26 samples (see
Table 4-21).
GS2970 3Gb/s, HD, SD SDI Receiver Complete with
SMPTE Audio and Video Processing
Data Sheet
47478 - 4
September 2012
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