HDCD feedback and information thread

[tmtomh]

The Pacific Micrsonics units allow for a number of HDCD features to be turned on or off during production. Some are "transient filters" aka a primitive version of MQA's claimed filtering, whereby the HDCD music file tells the HDCD-capable playback device what filter to apply based on how the file was encoded during production.

IMHO this is a useless feature. The only really useful/audible feature is Peak Extend, as noted above in this thread.

 

Not a whole lot of HDCDs are encoded with Peak Extend. The big notable ones are much of the Grateful Dead and Neil Young catalogues. If you don't have those, then you're likely to have a very small number of Peak Extend-enabled HDCD discs in your collection.

 

An HDCD can be ripped and then post-processed (also as noted earlier in the thread) to decode the HDCD content. The decoded HDCD stream is actually only 20-bit, but since that's an oddball bit-depth, the processing software always pads it out with 4 bits' worth of zeros, to produce a 24-bit file that can be played back pretty much any DAC.

 

All of this illustrates why HDCD is not very compelling technology. All that hassle for just 4 extra bits of bit-depth doesn't seem worth it - especially since exactly zero HDCDs with Peak Extend have musical content whose dynamic range exceeds the 96dB range of normal 16-bit CD. So what Peak Extend really did was enable engineers to peak-limit the mastering, because HDCD allowed the limited peaks to be recovered. If they just mastered the content at the proper volume to begin with, they wouldn't have needed to limit the peaks in the first place.

[tmtomh]

2 hours ago, vortecjr said:

I actually have a lot of HDCDs that have the Peak Extend feature enabled. This thread is not really about if it's worth it or not. The fact is that the content exists and needs to be dealt with properly in a collection.   

 

Well, how worthwhile it is on balance, given the overall number of HDCDs with Peak Extend on the market, is on-topic for a thread whose title includes the phrase "HDCD feedback."

 

But putting that aside for a moment, my comment clarified the bit-depth of the decoded HDCD stream, and that is substantive information. 

[tmtomh]

41 minutes ago, vortecjr said:

Again, that is not the purpose of this discussion. However many there are on the market they exist and I'm looking for comments...do you like them or not and under what circumstances. 

 

I didn't take exception to your comments on the technical aspects. 

 

Your first post asked, "what is your experience with it?" My comment was responsive to that. If you really want to keep going back and forth about what's on-topic, we can do that, but it seems beside the point. I certainly agree an HDCD thread is useful, and I'm glad you started one.

[tmtomh]

3 hours ago, vortecjr said:

Moving on...I noticed something interesting from the Foobar2000 scan of my library. The scan did not identify any hi-resolution content with the HDCD markers. I would have thought that the Reference Recording HRx tracks would be identified. According to the Reference Recording website, "Currently we record at 176.4 kHz/ 24 bits, with HDCD, and that is what we put on our HRx discs." 

 

https://referencerecordings.com/format/hrx

 

So this brings up some concern. The research I have done suggests that the HDCD recordings on CD are distorted unless you play them back via a hardware or software decoder. So naturally, I have to wonder if the HRx HDCD recordings on DVD are also distorted unless you play them back via a hardware or software decoder. More research is needed to know if the software decoder even works on the Reference Recording HRx tracks. 

 

This sounds very strange (not criticizing you; rather, referring to the Reference Recordings web site blurb). HDCD was designed for 16-bit recordings and I'm not aware of any existing decoder chips that look for HDCD data except in the 16th bit. In other words, even if Pacific Microsonics makes/made an ADC capable of applying HDCD encoding to a 24-bit data stream and burying the HDCD data in the 24th-bit, I don't think any consumer playback device could decode that.

 

The way the Reference Recordings blurb is written, it is not clear what they mean when they say they still currently use HDCD in the production of their high-res recordings. If they do, I would guess that probably means they use Pacific Microsonics ADC units - which happen to be HDCD capable - to digitize their recordings. But I would be very surprised if they enable the substantive features of it - Peak Extend and Low-level Range Extend. It would hardly be necessary with high-res content, since high-res provides more bits and more dynamic range than HDCD (24 bits vs 20 bits), and again, AFAIK no consumer playback software or hardware could decode those features if they were embedded in the least significant bit of a 24-bit stream.

 

In other words, one can use a Pacific Microsonics ADC for recording, and enable HDCD but not actually use any HDCD features - which means the resulting disc/file will trip an HDCD flag but there's no actual HDCD data or features in the digital content. In fact, I believe one can even use a PM ADC and simply turn off its HDCD features altogether, meaning one can record non-HDCD content using a PM unit. In that case, one might still say "we use HDCD" when they really just mean "we use PM equipment to digitize our recordings." I suspect one of these two scenarios is what happens with Reference Recordings' high-res material.

 

So that leaves the digital filters - both the filters used in the ADC during recording, and the digital filters built in to any HDCD-certified/capable hardware playback device. Part of HDCD's claim was that the digital filters built in to HDCD hardware players were special/high-precision/very good. And presumably they were considered to match up well, or be complementary, to the filtering used by the Pacific Microsonics ADC.

 

Software HDCD decoders/players do not, AFAIK, necessarily have such filters - or if they have multiple filters choices, I don't know that they automatically switch to HDCD filters when playing back files that were ripped from HDCD discs.

 

Finally, as to your concern about HDCD recordings on CDs being "distorted" unless played back on HDCD hardware or software, there are two sources of distortion:

1. Increased noise, because the HDCD data is buried in the 16th bit, meaning it's in the dither, and because HDCD data is by definition not random, it therefore makes the dither not totally randomized - and since the entire function of dither is to reduce the perception of quantization noise by randomizing it, HDCD encoding makes dither less effective. Again, this is not an issue with high-res content (because it hardly matters if it has a 23-bit effective noise floor vs 24), but theoretically an undecoded HDCD will have only a 15-bit noise floor, while a decoded HDCD will have a 20-bit noise floor. This is not distortion per se, but it does reduce S/N ratio.
2. Distortion of peaks if Peak Extend was used. Peak Extend is a reversible soft limiter - basically just like the widespread digital limiters used (and overused) in mastering today, except HDCD Peak Extend saves the original peak info and it can be restored during playback by an HDCD-capable device or software. So playing an HDCD with Peak Extend on a non-HDCD playback device will result in the peaks being limited. That's unfortunate, but it's not going to result in audible distortion in most cases, because the peaks are soft limited rather than hard-clipped: the peaks will be altered and their dynamics reduced, but the entire point of soft limiting is that it avoids audible distortion (unless the mastering engineer pushes the levels so hard and abuses the limiter so much that it can't round off the peaks properly).

[tmtomh]

1 hour ago, Miska said:

 

I have successfully ripped all my few (3 Mark Knopfler albums in total) HDCD encoded CDs into 24-bit AIFF using dbPoweramp and the HDCD decoder plugin. The plugin's gain setting just works exactly opposite way than what at least I understand from the description. So check the results afterwards in Audacity for correctness, that you don't get extra clipping! I needed to do the ripping twice because of this.

 

 

Interesting - I've never used the dbPoweramp/HDCD plugin combo. I've only used hdcd.exe in command line mode (in a Windows emulator on my Mac).

 

If I understand your comment correctly, it sounds like the HDCD plugin restored the limited peaks, but did not perform the accompanying - and necessary - reduction in overall gain (aka makeup gain) of the waveform. resulting in all those peaks going into clipping. Is that right? If so, I find that a little strange - to my knowledge, hdcd.exe automatically applies makeup gain (or I should say makeup attenuation) of 6dB to compensate for the louder peaks that occur when the full peaks are restored during HDCD decoding. I'd always assumed that this was just a hard-wired (so to speak) part of the HDCD spec and that any hardware or software HDCD decoder would properly adjust the gain as a matter of course.

 

So if you did it twice, it sounds like the 2nd time you changed a setting in the decoder plugin, to make it perform the gain reduction so the restored peaks would all be below digital max aka 0.0 and avoid clipping - yes?