HDMI audio: capabilities, formats, testing

1
HDMI audio capabilities, formats, testing

• Thomas Kite
• Director of Engineering, Audio Precision
• Beaverton, OR

2

• HDMI (high definition multimedia interface) is a
  standard for audio/video transmission between a
  source and sink over a single cable
• Provides for high definition video (up to
  19201080p _at_ 60Hz, 48 bits per pixel) and high
  resolution audio (up to 192 kHz, 24 bits, 8
  channels, uncompressed)
• Standard on most new consumer A/V equipment
• TVs, DVD players, Blu-ray players, PCs
• Camcorders, cellphones (soon)
• Latest specification is Version 1.3a (HDMI
  Licensing LLC, November 10 2006)

2
3
Signalling

• Three transition-minimized differential
  signalling data channels and a TMDS clock channel
  for video and audio
• Clock rate is (a multiple of) the video pixel
  clock rate
• Audio, metadata sent during video blanking
  intervals
• DDC (display data channel) for configuration
• Sink informs source of its A/V capabilities
• Optional CEC (consumer electronics control) bus
  for remote control information

3
4
How audio fits in

• HDMI has huge bandwidth (up to 10 Gbps) on the
  TMDS lines
• Needed for high resolution video
• Video transmission method identical to DVI
• Audio and metadata are contained in data
  islandspackets inserted into the stream during
  blanking intervals
• At high video resolutions, plenty of bandwidth
  for high resolution uncompressed audio, lossily
  or losslessly compressed audio, or 1-bit audio
  (DSD)
• Audio capabilities reduced at low video
  resolutions (lower sample rates, channel counts)

4
5
Audio support on HDMI

• HDMI 1.0 (December 2002)
• Linear audio 8 channels, 192 kHz, 24 bit
• Compressed audio IEC 61937 up to 6.144 Mbps
• HDMI 1.1 (May 2004)
• Added support for DVD-Audio
• HDMI 1.2 (August 2005)
• Added support for 8-channel DSD audio
• HDMI 1.3 (June 2006)
• Added support for high bit rate lossless formats
  Dolby TrueHD and DTS-HD Master Audio

5
6
Audio packets N/CTS

• The audio clock is not transmitted over HDMI
• Rather, it is derived at the sink end from the
  video clock
• Source computes integers N and CTS such that
• 128fs fTMDS_CLKN/CTS
• N is fixed for a given video and audio rate
  (table lookup)
• Source counts TMDS clocks per audio clock to
  determine CTS
• N, CTS transmitted in audio clock regeneration
  packet
• The sink regenerates the audio sample clock from
  the received fTMDS_CLK, N, and CTS values
• Asynchronous video and audio clocks, or audio
  clock jitter, cause CTS to change over time

6
7
Audio packets L-PCM/IEC 61937

• Formatted similarly to IEC 60958
• Allows for IEC 61937, which is built on IEC 60958
• 28-bit words up to 24-bit audio, plus VUCP
• 192-bit channel status block start also encoded
• Supports up to 8 channels of linear 24-bit audio
  at192 kHz sample rate
• Also supports IEC 61937 up to 6.144 Mbps (16-bit
  words at 192 kHz frame rate)
• Dolby Digital (max 640 kbps)
• Dolby Digital Plus (max 6.144 Mbps)
• DTS Digital Surround (max 1536 kbps)
• DTS-HD High Resolution Audio (max 6.0 Mbps)

7
8
Audio packets High bit rate

• Added for HDMI 1.3
• Supports IEC 61937 above 6.144 Mbps
• Used for lossless compression schemes
• Dolby TrueHD (max 18 Mbps)
• DTS-HD Master Audio (max 24.5 Mbps)
• Blu-ray discs have the capacity and bit rate to
  support these formats
• Many current implementations decode in the player
  and transmit L-PCM over HDMI (works with HDMI
  1.0)
• Some receivers have decoders built-in, e.g. Onkyo
  SR-606

8
9
Audio infoframe

• Audio metadata sent at least once per two video
  frames
• Some information is already in the audio stream
• IEC 60958 channel status bits (sample size,
  sample rate)
• IEC 61937 burst info (coding format)
• Audio infoframe adds extra information
• Speaker allocation for linear streams
• Level shift value for downmixing
• Downmix inhibit for DVD-Audio

9
10
What about jitter?

• Potential sources of jitter
• Jitter in the source audio clock
• Jitter in the source video clock
• Asynchronous audio and video clocks (causes CTS
  to change over time, usually over 2 or 3 values)
• Poor performance of sink PLL
• We found that the newest Silicon Image HDMI
  transmitter/receiver pair (9134/9135) is highly
  resistant to jitter
• Injecting even high levels of jitter into the
  source audio clock produced no measurable jitter
  at the receiver

10
11
E-EDID

• Enhanced extended display identification data a
  data structure supplied by the sink device
• Informs source of the sinks rendering
  capabilities
• Video resolution and interlacing
• Video frame rates and color space
• Number of audio channels and types of compressed
  formats
• Enhancement of EDID, devised by VESA for computer
  displays
• E-EDID is transmitted over the display data
  channel (DDC)
• When source and sink are connected, source powers
  E-EDID ROM inside sink and reads it using I2C
  commands
• Source is responsible for supplying data to sink
  in a form that the sink can handle
• Possible downconversion of video
• Possible decoding, downsampling, and downmixing
  of audio
• E-EDID is probably responsible for most HDMI
  problems

11
12
CEC

• Consumer electronics control shares control
  information among all HDMI-connected devices
• Offers convenience features
• One-touch play and record
• Automatic source switching
• Timer programming
• System audio control
• Single line connected to all devices in the chain
• Automatic address discovery
• Dozens of message types
• Specification is almost as long as HDMI spec

12
13
HDCP

• High-bandwidth digital content protection an
  authentication/ encryption scheme used by HDMI,
  DVI, and DisplayPort
• All HDCP-compliant devices contain 40 56-bit keys
• Source initiates authentication by sending 40-bit
  key selection vector (KSV) to sink
• Sink responds with its own 40-bit KSV
• If sinks KSV is on the revocation list (stored
  on media), source will not proceed with
  authentication
• Source and sink compute identical keys using
  counterparts KSV
• Pixels encrypted/decrypted with these secret keys
• Keys updated each video frame
• HDCP probably the next most common reason for
  HDMI trouble
• HDCP repeater problems (try connecting devices
  directly)

13
14
Testing audio over HDMI

• Optional module for theAPx 585 audio analyzer
• Generates linear, 8-channel, 24-bit audio
• Generates compressed audio (Dolby Digital, DTS,
  MLP, DTS-HD MA)
• Generates all standard video resolutions and
  frame rates
• Auxiliary video input
• Selectable HDCP encryption
• Analyzes 8-channel linear audio
• Audio/video monitoring

14
15
Summary

• HDMI has extensive audio capabilities
• 8 channels of linear audio at 192 kHz, 24 bit
• Legacy lossily compressed multichannel formats
• New lossily compressed multichannel formats
• New losslessly compressed multichannel formats
• HDMI has complex audio testing requirements
• Source devices must supply audio formatted
  correctly for sink, based on E-EDID
• Possible transcoding in source
• Many supported audio formats and sample rates
• Sink devices must be able to handle all audio
  formats specified in E-EDID

15
16
For more information

• Please visit
• http//ap.com/products/apx_hdmi.htm

16