GS2972 Datasheet, PDF (56/125 Pages) Semtech Corporation – 3G/HD/SD-SDI Serializer with Complete SMPTE Audio & Video Support

4.7.15 Audio FIFO - SD
Each input channel has a First In First Out (FIFO) buffer that can hold up to 52 samples.
Samples are added (written) to the FIFO as they are received from the audio inputs.
Samples are removed (read) from the FIFO as they are embedded in audio data packets
and audio extended packets.
After power up, reset or clear, the FIFO is in the start-up state where it will output zeroes
until it has accumulated the start-up count of 26 samples.
When the start-up state ends, the buffer operates as a normal FIFO, and expects to
receive an equal number of read and write operations over the period of five frames. At
the end of five frames, the FIFO still has 26 samples in the buffer.
When the FIFO does not receive an equal number of read and write operations, the FIFO
checks for the overflow and underflow conditions.
When a sample is required for embedding into a packet and the FIFO is holding less than
6 samples, the GS2972 prevents the underflow condition by repeating the last sample
without removing a sample from the FIFO. Therefore, a sample will be duplicated.
When an input sample is received and the FIFO has room for less than six more samples,
the GS2972 prevents the overflow condition by discarding the sample. Therefore, a
sample will be dropped.
If 28 consecutive samples are duplicated or dropped, the audio FIFO is cleared and
placed into the start-up state.
If the CLEAR_AUDIO host interface bit is set, the audio FIFO is cleared and put into the
start-up state.
When the detected video standard changes, the audio FIFO is cleared and put into the
start-up state.
The buffer size and start-up count can be reduced using the OS_SEL host interface field,
as seen in Table 4-11 below:
Table 4-11: GS2972 SD Audio Buffer Size Selection
Address OS_SEL[1:0]
00
01
10
11
Buffer Size
52 samples (default)
24 samples
12 samples
Reserved
Start-Up Count
26 samples
12 samples
6 samples
Reserved
GS2972 3G/HD/SD-SDI Serializer with Complete
SMPTE Audio & Video Support
Final Data Sheet Rev. 9
GENDOC-047479 September 2013
www.semtech.com
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