DisplayPort Intel® FPGA IP User Guide

ID 683273
Date 1/24/2022
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5.8.8. Audio Interface

The audio encoder is upstream of the secondary stream encoder. It generates the Audio InfoFrame, Audio Timestamp, and Audio Sample packets from the incoming audio sample data stream. Then, it sends the three packet types to the secondary stream encoder before they are transmitted to the downstream sink device.

You can configure the audio port for the number of audio channels required in the design. You can use 2 or 8 channels. Each channel’s audio data is sent to the txN_audio_lpcm_data port.
  • Channel 1 audio data should be present at txN_audio_lpcm_data[31:0]
  • Channel 2 audio data should be present at txN_audio_lpcm_data[63:32] and so on.

The IP requires a txN_audio_valid signal for designs in which the txN_audio_clk signal is higher than the actual sample clock. The txN_audio_valid signal qualifies the audio data on the txN_audio_lpcm_data input. If txN_audio_clk is the actual sample clock, you can tie the txN_audio_valid signal to 1.

The figure and table below illustrate the audio sample data bits and bit field definitions, respectively.

Figure 24. Audio Sample Data BitsThe packing format complies to both IEC-60958-1 and IEC-60958-3 standards.


Table 34.  Audio Sample Bit Field Definitions

Bit Name

Bit Position

Description

Audio sample word

Byte 2, bits 7:0

Byte 1, bits 7:0

Byte 0, bits 7:0

Audio data. The data content depends on the audio coding type. For LPCM audio, the audio most significant bit (MSB) is placed in byte 2, bit 7. If the audio data size is less than 24 bits, unused least significant bits (LSB) must be zero padded.

V

Byte 3, bit 0

Validity flag.

U

Byte 3, bit 1

User bit.

C

Byte 3, bit 2

Channel status.

P

Byte 3, bit 3

Parity bit.

PR

Byte 3, bits 4 - 5

Preamble code and its correspondence with IEC-60958 preamble:

00: Subframe 1 and start of the audio block (11101000 preamble)

01: Subframe1 (1110010 preamble)

10: Subframe 2 (1110100 preamble)

R

Byte3, bit 6

Reserved bit; must be 0.

SP

Byte 3, bit 7

Sample present bit:

1: Sample information is present and can be processed.

0: Sample information is not present.

All one-sample channels, used or unused, must have the same sample present bit value.

This bit is useful for situations in which 2-channel audio is transported over a 4-lane main link. In this operation, main link lanes 2 and 3 may or may not have the audio sample data. This bit indicates whether the audio sample is present or not.

When you configure the DisplayPort Intel® FPGA IP for 2 or 8 channels, you can transmit any number of audio channels fewer than or equal to the number of channels you selected.

To transmit 1 channel of audio over the IP configured at 2 audio channels:
  • You must configure the source audio register's CH_COUNT bits to 000b using the embedded controller.
  • You also need to set the SP bit to 1 and the other bits to 0 on the txN_audio_lpcm_data[63:32] signal. The IP performs 2-channel layout mapping for 1 and 2 audio channels, which requires the SP bit to be the same for all one-sample channels.
Figure 25. Typical 1-Channel Audio Flow Over 2-Channel Audio TX Core


To transmit 3-8 channels of audio, the IP performs 8-channel layout mapping. For example, to transmit 3 audio channels over the IP configured at 8 audio channels:
  • You must configure the source audio register's CH_COUNT bits to 010b using the embedded controller.
  • You also need to provide the data as shown in the figure below.
Figure 26. Typical 3-Channel Audio Flow Over 8-Channel Audio TX Core


The DisplayPort Intel® FPGA IP internally calculates the Maud based on a fixed (8000h) to generate the Audio Timestamp packet. The IP generates the Audio InfoFrame packet based on the information from the DisplayPort source audio registers: LFEBPL, CA, LSV, and DM_INH. The IP continues transmitting the Audio Timestamp, Audio InfoFrame, and Audio Sample packets even when the main video stream is no longer transmitting. When there is no video stream, the IP transmits an Audio Sample packet after each BS symbol, and transmits an Audio Timestamp and Audio InfoFrame once after every 512th BS symbol set.

The source automatically generates the Audio InfoFrame and fills it with only information about the number of channels used.

Use the audio channel status to provide any information about the audio stream needed by downstream devices.