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Advancing Audio Streaming, Part 2: Bit-Perfect Playback

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[This article is co-authored by David Blumenstein]

Bit-Perfect Playback…and why you should care

David Snyder: By the time you read this, the new Amazon Music HD streaming service, promising “High Definition” audio to the masses, will have been available in the US and several other markets for a month or two. The initial subscription offering hits the wallet $5 to $10/month lighter than comparable services from TIDAL and Qobuz.

But is Amazon’s offering really comparable? A library that’s 50 million tracks strong is impressive, but what concerns me is the quality of delivery via available platforms. Amazon may be betting that their new Echo Studio will bring to the uninitiated their first taste of high definition sound reproduction, but for long-time audiophiles like us, high-performance playback is nothing new; we’ve been doing it for decades.

After persistent cajoling from fellow Dagogo Reviewer David Blumenstein, my digital audio “partner in crime,” I reluctantly registered for Amazon’s 90-day free trial. I downloaded the desktop app for macOS, connected a USB DAC, and proceeded to experiment. To my astonishment and dismay, the application has no settings for exclusive mode or directly interfacing with an external DAC. This is bad because we don’t get bit-perfect playback.

David Blumenstein: It has been some time now that Amazon has launched its HD Music venture and they truly have chosen the path of least resistance, a nice way of saying they’re being somewhat lazy with their software. Granted, the audiophile community is niche compared to the mainstream, but then again is the mainstream crying out for super high resolution? Do they even know what that means? The audiophiles for the most part do, yet Amazon has seen fit to offer up a thin veneer, an overlay if you will, leveraging device’s system audio in lieu of providing an “audiophile” layer between their software and external DACs.

I expected more from Amazon in this regard. Best thing about Amazon’s foray, its dipping its big toe into the streaming music service morass, is that it had Qobuz drop the price on their service, which for some was an obstacle. With Amazon’s HD Music, I can’t be bothered having to remember to go into my device’s mixer and reset the bit-rate and depth. That being said, for use on the go with my iPhone and wireless Grado GW-100 Bluetooth headphones, I couldn’t care any less as I’m not going to be listening under ideal environmental conditions.


So, what is bit-perfect?

The term bit-perfect is used a few different ways, but I’m using it here to describe the transfer of digital audio data from one point to another with absolutely no changes. After all, this is a crucial feature that sets digital apart from analog…zero generation loss. Take a digital file and copy or transfer it, then take that copy and transfer it again. Repeat this 10,000 times, and what you have at the end will be bit-for-bit identical to the samples in the original.

The rub with Amazon Music HD is that, currently, there’s no way for it to bypass the O/S mixer on supported platforms, including Windows, macOS, Android, and iOS. The O/S mixer is a piece of software that enables you to hear beeps and boops from email and calendar notifications over whatever music you are playing, so that you don’t miss your next meeting or fail to respond to that proposal from your boss. This is great for background in an office setting, but for audiophiles, it will not do.

The main issue here is not the beeping interruptions to our music but what the O/S mixer must do to support them in the first place. It’s not possible to mix digital audio with different sampling frequencies without messing up pitch and timing, so O/S mixers resample the music, beeps, or both to a common sampling frequency. Resampling is done in real time and is rarely of the highest possible quality, so sound quality takes a hit. The O/S mixer usually includes volume controls and DSP features for both beeps and music, which further mangles the original samples, even if no beeps happen to play through during the song. Some of these features can be disabled, but as long as the samples are passing through the O/S mixer, it’s difficult to know if they are truly arriving at the DAC’s input with zero modification.

David Blumenstein: This is why I don’t bother with either Qobuz’s or TIDAL’s native software, mind you Tidal gets the nod because it affords users more customization options with their particular DAC—exclusive modes comes to mind as do volume settings and MQA passthrough. Still, I much prefer using Roon and/or Audirvana as the front-end to both streaming services. It would be really nice if Amazon would strike up some sort of integration deal with either of the two (both would be best) to place its venture squarely in the frame.

Sure, I could, if so inclined, purchase hardware that integrates Amazon HD Music, but you know what, and I reckon I’m like most of you out there, I don’t particularly care to have my hand forced and to make a purchase for lack of a better option.


Why is this so bad?

Computers and smartphones have been silently resampling and mixing music with application sounds for many years, and no one has cared. Most people are not even aware that this is happening. But most people don’t have their computer or smartphone connected to a high-performance playback system. Once the resolution of the playback system reaches a certain point, you begin to care about audio quality and what happens to your music along the path from the media source to your speakers. A digital audio system is fundamentally flawed if it cannot reliably support bit-perfect playback.

David Blumenstein: I go back to one of my earlier points, I just don’t care enough when I’m going mobile, out and about, I will splash for decent quality and comfortable over-the-ear headphones, so there is that. This is about context and priorities. You can be damned sure that this mobile digital audio system in my house is more than fundamentally flawed, it is rendered useless as an option when listening with my home audio systems.

CD players were not bit-perfect.

Eeeek! It may come as a surprise to some, but the promise of “perfect sound forever” was never fully guaranteed by the common CD player. During real-time playback, there was no time for the laser to re-read bad frames, so error detection and correction routines were engaged. In some cases, redundant data in adjacent frames could be used to repair the damage from a misread, but often the player was left to guess at the lost samples, using interpolation or similar techniques to recover from the error. The show must go on! I’ve always been curious to know how many times error detection and correction are engaged during playback of a typical CD, but I’ve yet to encounter a CD player that exposes this information.


Computer Audio and Streaming to the rescue

The draw of computer audio to many nerds like me was the idea that we could finally hear what our music sounded like without errors. Unlike compact disc digital audio, computers use well-defined data structures and protocols to ensure that data is transferred reliably and without error. Many millions of US citizens electrically file their tax returns over computer data networks with no concerns over digits being flipped in their tax bill or refund due. The data is bit-perfect.

While reading (ripping) audio from a CD to a digital file in a perfect and verifiable way is a non-trivial process, made easier through applications like dBpoweramp and XLD, once the music is at rest on the internal storage of the computer, a digital audio playback system must be able to deliver each sample with perfect precision from the file to the input of our DAC. Amazon Music HD does not currently offer a way to do this, but perhaps it will someday.

David Blumenstein: I too, as a nerd and proud to self-identify as one, welcomed and continue to welcome computer audio and streaming services There is a distinction between the two and that devolves into the terminology kerfuffle over how the “streamer” as a device came to be. I rest easy in the reality of a bit-perfect digital audio continuum, hence my personal frustration with Amazon HD Music at the present. The powers that be over there should know better. If it ain’t bit-perfect, it’s just a bunch of bits.


Copy editor: Dan Rubin

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3 Responses to Advancing Audio Streaming, Part 2: Bit-Perfect Playback

  1. Ant says:

    Hi, great article, can you advise is this still the case that amazon music HD is still not bit perfect when used via the iOS app connected to an external Dac via camera connection kit ? Thank you

  2. Matthew says:

    I was looking forward to your comparison of the Allo USBridge with it dedicated audio board to the regular Pi4 as suggested in Part 1. Did you eve have a chance to do the review?

  3. David Snyder says:

    @Ant, when I re-tested a few months ago, Amazon Music HD was still not bit-perfect, even on iOS. If anyone has found differently, please let me know.

    @Matthew. I’ve since picked up a ZEN Stream from iFi Audio and found that it sounds better than the Allo USBridge Signature and VitOS. The differences are small and are more pronounced with some DACs, like my Khadas Tone2 Pro than with others, like my Gustard X16. If you are looking for a streamer that works well with Roon out of the box, the iFi ZEN Stream is a great way to go.

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