MMC-2 Commands

chansen at ayre.com chansen at ayre.com
Sat Jan 19 17:07:04 PST 2002


* From the T10 Reflector (t10 at t10.org), posted by:
* chansen at ayre.com
*
To whom it may concern:

We have found an amibiguity in the MMC-2 specification that causes
problems when playing back HDCD-encoded audio discs. Here are further
details:

1) There is a technology used in many audio CDs called "HDCD", which
stands for "High-Definition Compatible Digital". This is a process
used to improve the sound quality of digital audio, whereby the audio
signal is compressed before storage (in level, not in terms of data
bandwidth), and then un-compressed upon playback.

The clever (patented) part of this process is that instructions are
stored within the audio stream that allows the un-compression to
exactly mirror the compression, so that any unwanted artifacts are
completely avoided. In order to properly play back these encoded
discs, the data retrieved must be bit-for-bit accurate. Otherwise, the
process will not function properly, and the disc will be played back
with reduced fidelity.

More information is available on their website:

www.hdcd.com

This technology is used on thousands of CD titles from major recording
artists on all major labels, due to the improved sound quality that is
possible. It became so popular that Microsoft bought the company and
its technology about a year or so ago. (As you know, Microsoft is
aggressively expanding into digital entertainment areas, including
audio, video, games, et cetera.)

2) The MMC-2 specification for ATAPI drives has section for Audio
Playback Parameters under the Mode Select commands. In this section is
a method for changing the volume level of the audio playback. This
affects both the analog outputs and the digital audio outputs of the
ATAPI drive. This information can be found in ANSI NCITS 333-2000 (the
MMC-2 spec) on page 112:

==========

ANSI NCITS 333-2000

The Output Port Volume Control indicates the relative volume level for
this audio output port. The value used is specified as an attenuation
of the normal volume level. A value of zero indicates the minimum
volume level (Mute), and a value of FFh indicates maximum volume (No
attenuation) level. It is recommended that the MUTE and volume
functions should be supported on a per channel basis. The attenuation
used shall be as specified in Table 132. All values not shown in the
table shall be valid, with the attenuation selected by interpolating
using the known table values. It is recommended that the Logical Unit
support at least 16 volume levels. The actual attenuation levels for
any given Binary attenuation value shall be given by the following
equation: 20 Log (Binary Level / 256) Note: Audio channel volume
control regarding channel selection of MUTE vs. Volume Level setting
of 0. It is recommend that Logical Units allow the setting of the
Channel Selection fields to MUTE and also allow the setting of the
Volume Level field to 0. It is up to the Logical Unit to determine how
to shut off the volume, either via muting circuitry or via the volume
control.

Table 132 - Attenuation Levels for Audio

Binary Level Attenuation
FFh 0db (0n)
F0h -0.52
E0h -1.12
C0h -2.45
80h -5.95
40h -11.9
20h -17.8
10h -23.6
0Fh -24.1
0Eh -24.6
0Ch -25.9
08h -29.1
04h -34.2
02h -38.6
01h -42.1
00h Mute (Off)

==========

Unfortunately, there is an ambiguity in this specification. Hex "FF"
is decimal 255. Therefore, the maximum volume level from a drive that
literally complies with the formula given in this specifcation is 20 *
log (255 / 256) times the actual signal on the audio disc. This
represents an attenuation level of -0.03 dB. This would normally be
insignificant, but it does represent an alteration of the data present
on the disc. On the other hand, if the drive uses the values given in
the table, there is no attenuation or alteration of the original
signal.

3) When an HDCD disc is played on an ATAPI drive that performs the
digital attenuation as described above, the audio data from the S/PDIF
digital audio output has been altered. Therefore the HDCD coding
information is lost, and the disc will not be played back with its
full fidelity.

4) Some ATAPI drives do not have this problem. Presumably, they either
do not have the attenuation feature at all, or if they do, then full
volume (hex FF) is set so that it transmits the audio data
un-attenuated rather than multiplied by 20 * log (255 / 256).

5) The Pioneer DVD-ROM mechanism DVD-106S does perform this
attenuation, and therefore provides improper playback of any HDCD
disc. We have tested two different generations of the DVD-106S drive
with identical results:

Unit #1 -- Code DVD-106S
Mfd. -- January 2001
Serial Number -- AAMS012034WL
Firmware Rev. -- 1.14

Unit #2 -- Code DVD-106S/2
Mfd. -- December 2001
Serial Number -- ALAR166845WL
Firmware Rev. -- 1.22

I am under the understanding that you have some input as to how these
standards are written. If you could please address this issue, we
would deeply appreciate it. Thank you for your kind assistance.

Best regards,
Charles Hansen
Research Director

Ayre Acoustics, Inc.
2300-B Central Avenue
Boulder, CO  80301
www.ayre.com
(303) 442-7300 Voice
(303) 442-7301 Fax 

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