Article - Issue 84, September 2020
HOW DOES THAT WORK? - MQA
MQA technology captures and authenticates the sound of the original master recording in a file small enough to stream at high resolution, allowing listeners to feel that they are in the studio with the performer.
How we listen to music has transformed significantly over the years: now most of us can access songs whenever and wherever we want to listen to them. In general, downloaded audio or streaming services offer listeners standard, compressed files that weren’t produced by the artist themselves but created by third parties from studio quality master digital files, often leading to inaccurate representations of the audio.
The graph shows the information capacity of 192kHz, 24bit PCM and the audio within it. Region A is the conventional audio information; Region B, higher in frequency, manifests temporal microstructure in the sound; Region C carries noise consequential on the transmission sampling rate
Launched in 2015, Master Quality Authenticated (MQA) digitally captures and stores original master recordings as files that are small and convenient enough to download or stream. It captures archives efficiently, distributes music with the highest possible sound quality and then optimises it for each playback device. This end-to-end system removes unintended artefacts of the technology employed in real-world recording, distribution and playback.
Although commonplace, converting audio to digital (and back) is imperfect and can limit sound quality. Conventional digital (PCM) is convenient for signal processing but inefficient for storing audio information. For example, lossless compression – which allows the original data to be reconstructed – can store an archive of CD audio in half the space, at an average data rate of 750kbps. However, when higher sampling rates and more bits are used to improve resolution, the PCM file becomes unwieldy. It can represent sounds that are quieter than atmospheric noise or at inaudibly high frequencies. With high resolution, the appropriate audio information captured can increase to around 1Mbps, but a lossless file may use 5 to 10Mbps to deliver it, violating a key engineering principle that the channel capacity should match the signal.
MQA solves these problems in two ways, starting with ‘encapsulation’, which identifies the audio information in the recording; preserves the temporal microstructure; and avoids noise-modulation artefacts. Next, a process called ‘music origami’ makes the file smaller by ‘folding up’ extended-resolution information, while packing it underneath the standard audio, below the level of silence. This fits the audio information into a small lossless PCM file of low data rate (typically 1.2Mbps) that is more efficient to stream or download, yet higher in quality than prior methods.
Finally, signalling is added, inaudibly embedding metadata about the recording, playback instructions for the decoder, and a provenance signature. This is completely removed by MQA decoders but remains accessible even if a 24-bit stream is cut down to 16 bits (for example when played over Wi-Fi). Even in these circumstances, the MQA system preserves a large part of the music’s temporal microstructure.
This master file, which contains the entire performance, can be checked by the producer for several playback scenarios and authenticated for accuracy on playback. MQA can be played on a Hi-Fi, a smartphone, a portable player, in the car, in a PC, on a Wi-Fi speaker or Bluetooth headphone, and is also backward compatible, giving higher than CD-quality even on a device with no decoder.
Products with a full MQA Decoder unfold the file to deliver the highest possible sound quality. At this level of playback listeners hear what the artists created in the studio.
Bob Stuart, creator of MQA, has been awarded the Royal Academy of Engineering’s Prince Philip Medal for his exceptional contribution to audio engineering, which has changed the way we listen to music and experience films. He is a graduate of the University of Birmingham and Imperial College London, where his studies included psychoacoustics and electronic engineering. Bob is an expert in audio coding and was the brains behind MLP (the audio technology at the heart of DVD-Audio, and now part of the Blu-ray Disc specification). And he is, crucially, a dedicated lover of music.