I have been doing a lot of reading these last few weeks. There is quite some discussion on fora and audio news sites on Meridians latest invention called MQA. But what is MQA exactly and what is Meridian not telling us? This is what we will investigate in this article.
This article is based on info found on fora and audio news site, but mostly on technical descriptions from patents. I have not had the pleasure to listen to MQA myself, so I can not confirm nor refute any claims made on the quality of the sound.
2 versions of MQA
Where do we start? I came across this very interesting screenshot of the system Meridian is using to demo MQA on the CES. Besides the fact that they obviously demo MQA by comparing it to 128kbps MP3, we see that MQA comes in 2 flavours: Download at 44.4/48kHz 24 bits or Streaming at 88.2/96kHz 16 bits. We also notice that the download version is substantially bigger than the streaming version, because it has a higher bitrate.
Actually the 2 versions contain exactly the same data, however the download version is also backward compatible with legacy systems. Using a complex system the 96/16 PCM data is losslessly compressed into the 48/24 PCM data (same goes for 88.2/16 into 44.1/24). The top 16 bits of this data is playable as regular ‘CD quality’. This all follows from the patent named “Doubly Compatible Lossless Audio Bandwidth Extension”.
I remember reading somewhere that development took 5 years, so I guess at that time it seemed necessary to provide a backward compatible download version. To me it now seems a waste of space and CPU power, since the format is inefficient and unnecessarily complex compared to direct lossless encoding. Also nowadays there is no need for the backward compatibility as most or all media players handle 96/16 files just fine. I prefer the streaming version as download version, so there is no need for the download version in my opinion.
Just a PCM stream
So MQA is just a 88.2 or 96kHz PCM stream with 16 bits resolution. According to Meridian and as mentioned in the patent this is enough to satisfy audiophile requirements. In Mr. Stuarts 2004 AES paper “Coding High Quality Digital Audio” he concludes (page 19) that to capture all hearable frequencies we need 58kHz sampling rate with a 14-bit representation with appropriate noise shaping. MQA clearly exceeds this minimum requirement.
However from more recent research it turns out that although humans can not hear frequencies above 20kHz, they are sensitive to timing of sounds to about 10 microseconds. So first you notice the arrival of a sound (quick change in air pressure, a very high frequency) and later on you actual hear what sound it is. To preserve this timing info in the audio signal 96kHz is therefor not enough, we actually need 192kHz. MQA does not seem to take this into account?
Since with very high frequencies it is all about the timing (and amplitude) and not about the actual frequency, Meridian found a way around this. The solution is described in the patent named “Digital Encapsulation of Audio Signals”. In the presentation of MQA Mr. Stuart states that the information of the 192kHz signal is carefully folded into the 96kHz signal. The truth is that this automatically happens if you down sample without a lowpass (or brick wall) filter at the Nyquist frequency (half of the signal frequency). For instance if you down sample from 88.2kHz to 44.1kHz signal, 23kHz will be mapped onto 21kHz, 24kHz onto 20kHz and so on.
With the frequencies of the above example this is considered to be a big problem. However the higher frequencies (above 76kHz) of the 192kHz signal (above 68kHz for a 176.4kHz signal) have far less energy than the lower frequencies (below 20kHz and within human range). So in this case it should be no problem. And to avoid contamination of the lowest 7kHz range (to which the human hearing system is most sensitive), Meridian deploys a small 6 tap FIR filter to attenuate the upper frequencies of the 192kHz signal, since they will be mapped on the lowest frequencies after folding.
Decoding of the 96kHz MQA stream to the original 192kHz signal is simply performed by calculating in-between samples by averaging the 2 neighbouring samples. Besides that a small correction filter is applied to compensate for the filter used at the encoding stage. The idea is that this up sampling process should happen in the DAC or just before the DAC.
So there you have it: MQA is just a 192kHz signal folded into a 96kHz 16 bits PCM stream. Is that all or are we missing something?
The missing link
I have seen claims of people that have heard MQA that it sounds better than a 192/24 version of the same song. Based on all information we gathered above, this is not possible. In the best case it should sound just as good. Also Meridians co-founder Mr. Stuart claims MQA provides a transparent path from the original analog sound at the microphone to the analog sound at your home. We may find a clue in the above mentioned patent on Digital Encapsulation of Audio Signals. The very last claim (#38) states that the filter response is determined in dependence on information received from an encoder. In the notes at the end of the patent is referred to Pacific Microsonics, the creators of HDCD.
So most likely information on the used ADC in the mastering stage is hidden in the PCM stream, probably encoded in the LSB (least significant bit) as done with HDCD. This information is then used in or just before the DAC to compensate for any distortion caused by the ADC. The information is possibly the impulse response of the (lowpass/brickwall) filter used. Meridian was already compensating for this kind of distortion using its apodizing filter. Also possible would be information on the jitter, especially when it is data correlated.
Strangely enough Meridian does not communicate the above information, although it seems the only explanation to achieve better than 192/24 sound quality. It also seems to be the unique selling point of MQA, certainly to audiophiles. But somehow the marketing department thought they should present it as the next generation MP3. And maybe that is just the way it might work to sell it to the mainstream audience.