DisplayPort uses Secondary Data Packets (SDPs), which are transported over the Main-Link that are not main video stream data. This allows it to carry audio and video simultaneously. The VIP supports audio transmission both in the original mode as defined in the specification as well as just as any other SDP being transmitted.
Background of Audio Stream in DP
An audio stream is composed of the following components:
- Audio_Stream SDPs
- Audio_TimeStamp SDPs
- Audio INFOFRAME SDPs
- Partial audio time stamps within VB-ID
An Audio_Stream SDP includes the audio stream itself and some attribute information such as audio coding type and channel count. Depending on the coding type of Audio_Stream SDP, it may contain status information about the parameters of the audio stream.
Audio_Stream SDP over Main-Link for 8-Channel Layout Audio
An Audio INFOFRAME SDP is used for transferring audio stream attributes in a separate packet. Depending on the Audio_Stream SDP’s coding type, there is some overlap between the information carried by Audio_Stream SDP and that carried by Audio INFOFRAME SDP. Whenever there is an overlap, the information carried by the Audio_Stream SDP takes precedence.
An Audio_TimeStamp SDP is used for audio clock regeneration to restore the audio Primary clock frequency that is needed for further audio processing.
An Audio_TimeStamp SDP and Audio INFOFRAME (non-Basic Audio) SDP shall be transmitted once per frame and should be transmitted during the main video stream’s vertical blanking period. Audio_Stream SDPs shall be transmitted during the main video stream’s horizontal and/or vertical blanking periods.
Additional signals, such as “the audio mute” signal for disabling audio, are carried in the VB-ID byte transmitted next to each BS control link symbol.
An audio stream is a continuous (i.e., isochronous) stream of audio samples, each of which may contain several channels of audio signals at a pre-defined sample frequency (Fs). The sample frequency is typically within the range of 32 to 192 kHz for L-PCM Audio, L-PCM 3D audio, and non-HBR compressed audio. The sample frequency of HBR audio is within the range of 256 to 1,536 kHz. The sample frequency of One Bit and DST audio is within the range of 2,048 to 24,576 kHz.
Audio_Stream SDPs Transfer with No Video or During Video VBlank Period
Audio_Stream SDPs Transfer with Video During Video VActive Period
Audio Frame Format
A frame is uniquely composed of two sub-frames. In 2-channel operation mode, the samples taken from both channels are transmitted by time multiplexing in consecutive sub-frames. The first sub-frame (left or “A” channel in stereophonic operation and primary channel in monophonic operation) normally starts with preamble “M”. However, the preamble changes to preamble “B” once every 192 frames to identify the start of the block structure used to organize the channel status information. The second sub-frame (right or “B” channel in stereophonic operation and secondary channel in monophonic operation) always starts with preamble “W”.
Frame Format
Audio Format Change
The DPTX should start transmitting an audio mute signal prior to the audio format change by setting the AudioMute_Flag bit in the VB-ID. An audio format change is caused by a change in any of the following:
- Between the compressed and non-compressed audio
- Between different coding types
- In the sampling rate
- In the number of channels
Audio_Stream SDP Structure
The audio stream payload data structure has seven configurations:
- or 2-channel L-PCM Audio
- to 8-channel L-PCM Audio
- or 2-channel One Bit audio
- to 8-channel One Bit audio
- to 16-channel L-PCM 3D Audio
- 17- to 32-channel L-PCM 3D Audio
- DST audio
Speaker Mapping
Byte 4 of the Audio INFOFRAME SDP, as defined in CEA-861-F, contains information that describes the mapping of speaker locations into the audio transport channels. 
Speaker Placement
Channel-to-Speaker Mapping
Channel position in the DP stream with more than two channels should exactly correspond to CTA-861. This means that gaps between channels can be present.
Channel-to-Speaker Mapping of 3-Channel L-PCM Audio
Audio Stream Verification Challenges and Solutions
The different audio types supported by DP present several challenges.
At the transmit side, the user needs to provide the header bytes, data bytes and the AudioChannels array. VIP will internally handle:
- Packing of the audio data bytes into the corresponding channels
- Padding of zeroes for unused channels
- Setting the various status bits in byte 4 of each channel
At the receive side, VIP has several checks to ensure that the decoded SDPs are as expected.
- Decodes and validates the status bits based on the complex rules defined in the DP standard, IEC 60958 and CTA 861-F standards
- Uses the frequency derived from Audio TimeStamp SDP to validate if the expected number of audio frames are received.
Summary
Cadence VIP Display Port supports the transmission and reception of the various audio coding types defined above. It also supports the entire range of channel count. Validates if the audio stream packets are transmitted at the desired frequency based on the audio sampling rate defined in the associated AudioTimeStamp SDP.
More Information
Cadence has a very mature Verification IP solution. Verification over many different configurations can be used with DisplayPort 2.1 and DisplayPort 1.4 designs, so you can choose the best version for your specific needs. The DisplayPort VIP provides a full-stack solution for Sink and Source devices with a comprehensive coverage model, protocol checkers, and an extensive test suite.
More details are available in the DisplayPort Verification IP product page and Simulation VIP pages. If you have any queries, feel free to contact us at talk_to_vip_expert@cadence.com.