S/P DIF, BD and AACS
I got a call today from someone in the Midwest today asking about the FREE files that we make available through this site. He wanted to know how to acquire them and then how to make sure that he was able to hear in full 96 kHz/24-bits in his system. It reminded me of an email that I had received from Dr. David Robinson of Positive Feedback regarding a problem that he was encountering while trying to play some of our music.
It turns out that the copy protection scheme used on Blu-ray discs and hardware, called AACS (Advanced Access Content System), has a automatic downconverter on the S/P DIF coaxial digital output. Not too many people know about this. It turns out if you want to get the full HD-Audio out of a BD player you MUST use the HDMI connection. This is a requirement of the organization in charge of the BD specification at the request of the major record labels. Disc replicators are not allowed to press disc without the AACS coding in place.
About a year ago, Dr. Robinson had written:
“The stereo-only mix is supposed to be at 96kHz/24 bit, but my Bel Canto DAC 3.5 (their current reference) says that the feed via S/PDIF is only 48kHz. This is very definitely the “stereo-only” setting…I can hear the difference quite clearly between the surround mix and this one…but I’m not seeing 96/24. The Bel Canto DAC goes to 192/24 quite readily; any idea of what’s happening here?”
Mark, I’ve got a number of other 96/24 files and recordings here; I’ve never had a problem with the Oppo S/PDIF passing a 96/24 file. The Bel Canto definitely comes up at 48kHz.
I just loaded a known-good 96/24 disc, and got 96kHz on the Bel Canto.
I went back to the AIX, loaded it again, specified two channel in the opening menu…I could immediately hear the difference in the playback…then hit “play all.”
The Oppo indicates 2 channel PCM in the display window, but the Bel Canto still indicates 48kHz.
Puzzling…I don’t remember seeing this before, except with some DVD-As that weren’t delivering what the package promised (packaging error).
When we play this disc into the video monitor via HDMI the display clearly says 96 kHz/24-bits but his Bel Canto is saying 48 kHz. We don’t have a DAC or preamp with a display that will show the sample rate when plugging via S/P DIF but I figure perhaps he has the BDP-95 settings wrong.”
So I passed along the message to my friends at Oppo Digital and they replied:
The problem is caused by the AACS encryption and copy-protection scheme mandated for all Blu-ray discs and players. When you send the disc for replication, you must purchase an AACS key for the disc. Blu-ray Disc Association does not allow any disc to be produced without the AACS protection. On the Blu-ray player side, every player must conform to the AACS specification, which mandates that output via S/PDIF must be down-sampled to <= 48kHz and down-rez’ed to <= 16-bit. This is because the S/PDIF is an “unprotected digital output” without any encryption. With our BDP-93/95, the HDMI output can send digital audio to any compatible A/V receiver at up to 192kHz/24bit resolution. If the analog output is used, its digital signal before the D/A conversion is also up to 192kHz/24bit. We recommend users to utilize either HDMI or analog audio output (especially for the BDP-95) when listening to high resolution music such as the AIX Blu-ray, SACD and DVD-Audio.
It’s strange but true. This creates problems for certain users that want to connect the S/P DIF output to a digital crossover, as the customer in the Midwest wanted to do. I’m guessing that there a lot of audiophiles that are listening to 48/16 bit files via S/P DIF that don’t know it.
