Sections 126.96.36.199 and 188.8.131.52 of the HD Audio spec describe the scenario of over-subscription of SDO
and SDI bandwidths. It indicates that the final stream on the link is terminated prior to completion, and
further behaviour is undefined.
It is possible that the SDO line could be oversubscribed, if all streams are active at their maximum
sample rate, and word length. It is the responsibility of the Controller to handle this situation, and
determine what is put on the SDO line during oversubscription. The WM8850 will produce unsolicited
responses from the converter nodes that have not received the stream packets that they have been
The WM8850 invokes a protection mechanism, should a bandwidth over-subscription situation arise.
The mechanism monitors the sample rate, and sample-size settings of a stream to detect
If the SDI line is over-subscribed, due to software configuration or configuration from the recovered
S/PDIF sample-rate, the stream (or streams) assigned the highest Stream ID tag value, will be
dropped, and an unsolicited response issued from the stream converter whose stream has been
A stream will be restarted on a re-issue of a non-zero stream ID. However, should the bandwidth
problem still be present, the stream won’t start and an unsolicited response will be re-sent.
Although there is a protection mechanism in the WM8850, it is the responsibility of the user to identify
and rectify bandwidth problems.
SOFTWARE FORMATTED (RAW) S/PDIF
An S/PDIF sub-frame consists of a preamble, 24-bits of audio data, and four control bits. In Software
Formatted (Raw) S/PDIF mode, all 24-bits of the audio data, plus the 4 bits of control information,
plus a 4-bit encoding of the preamble (taking the last 4-bits of the 8-bit bi-phase encoded preamble)
are transported across the link. Software Formatted S/PDIF mode is selected by setting the Bits
register in the Stream Format Verb to 4h (i.e. 32-bits) and setting the Type register to 1.
SOURCE SYNCHRONOUS INPUT - S/PDIF RX
Section 5.4.3 of the HDA specification describes how source synchronous inputs should be handled.
Note: Source synchronous handling is only required when SRC1 is bypassed.
When the incoming sampling rate is slightly higher that the nominal stream sampling rate extra data
transmission is required. The HDA specification states “the CODEC may insert one or more
additional complete sample blocks on the link, either within its normal allocated stream packet
(making it larger than the nominal allocated size) or by creating a second stream packet within the
The WM8850 implements the first option, and will insert additional samples (in left/right pairs) within
the stream packet. The Data Length field within the Stream tag will be updated to show that the
stream packet contains additional samples.
If additional samples cannot be accommodated on SDI due to bandwidth problems, the additional
samples are dropped, and an unsolicited response is issued from the S/PDIF Rx Converter Node with
EF_STREAM_DROP set to 01h.
PP, April 2011, Rev 3.2