Home Computer Audio Asylum

Music servers and other computer based digital audio technologies.

RE: Sounds a bit like HDCD

The number of bits required to represent a signal at a certain accuracy depends on the bandwidth and the resolution. With regular PCM all frequency bands are given the same bit depth up to half the sampling rate. However, it is possible to reorganize the original signal into multiple frequency bands and encode these separately, then take the resulting pieces and get back the original signal. (In some cases, the exact same original signal, but I don't know if/when MQA does this.) This basic ability then allows the separate bands to be allocated different resolution by some clever signal processing techniques that have been used since the 1970's. By reducing the bits allocated to one band, bits are available to provide increased allocation to other bands. So, for example, if one sends regular 44/24 PCM one uses all of the available bits equally for frequencies from 0 to 22050 Hz, and there are no bits available to encode frequencies from 22050 to 44100 that would be available in an 88/24 carrier.

MQA robs bits from 0 to 22050 Hz and uses these to provide some resolution in the range 22050 to 44100Hz. This (supposedly) can be done in such a way that no distortion is introduced, just random noise that is not correlated with the music. The noise above 22050 can be fairly loud (supposedly) and still not be perceived.

This process of robbing Peter to pay Paul doesn't do anything to the sound quality that started in 88/24, but it could (possibly) produce better use of a channel capable of running at 44/24 but not 88/24. However, this analysis gives an unfair comparison of the actual bandwidth requirements for the 88/24 and the 44/24 recordings, because it doesn't take into account the possibility of lossless compression to these bit streams. It turns out (and can be seen by looking at example files on the 2L test site) that the amount of FLAC compression to 44/24 PCM is significantly more than the amount of FLAC compression to the 44/24 MQA encoded streams. Comparing a FLAC encoded 88/24 stream with the FLAC encoded 44/24 MQA stream shows that the entire MQA process hasn't really saved that many bits, but it has messed with the sound based on questionable psycho-acoustic research.


Tony Lauck

"Diversity is the law of nature; no two entities in this universe are uniform." - P.R. Sarkar


This post is made possible by the generous support of people like you and our sponsors:
  Herbie's Audio Lab  


Follow Ups Full Thread
Follow Ups
  • RE: Sounds a bit like HDCD - Tony Lauck 02/12/1611:08:31 02/12/16 (0)

FAQ

Post a Message!

Forgot Password?
Moniker (Username):
Password (Optional):
  Remember my Moniker & Password  (What's this?)    Eat Me
E-Mail (Optional):
Subject:
Message:   (Posts are subject to Content Rules)
Optional Link URL:
Optional Link Title:
Optional Image URL:
Upload Image:
E-mail Replies:  Automagically notify you when someone responds.