Streaming Audio Quality — An Overview

Author’s note: creating the definitive streaming audio quality guide is difficult, partly because services change frequently, but also because there’s a remarkable lack of good information out there. Few services clearly specify what systems they use, and many reviews of services done by magazines and websites in the past are just plain wrong. This article is as accurate as I can make it, but if there’s anything I’m mistaken about, please let me know.

Introduction: how we measure quality

There are two main variables that affect the quality of the sound you hear on a streaming music service:

1. The codec used to compress the audio
2. The bitrate, or amount of compression applied to the audio

Codecs

The word CODEC is short for COmpression & DECompression. CD quality music takes up a lot of bandwidth, so it needs to be compressed into a smaller space to be transmitted, then decompressed so you can hear it correctly. The most well-known codec is MP3, but there are many others.

Codecs fall into two main types: Lossy and Lossless.

A lossy Codec means that the Codec throws away some of the audio information during the compression stage, and so it can never sound as good as the original. Most audio Codecs are Lossy.

A Lossless Codec is different: it doesn’t throw away any original audio information when it compresses, it just repacks it into a smaller file space, and so when it is decompressed it is exactly the same quality as the original audio. So Lossless audio will always sound better than Lossy, but with the disadvantage that it requires a lot more data.

The main Codecs used today by streaming services are (in order of perceived quality, worst to best):

1. MP3
2. Ogg Vorbis
3. AAC
4. FLAC

MP3

Oldest Codec in general use, very widely supported but not great quality, especially at low bitrates. It’s an inefficient Lossy codec, which means that a significant amount of the original audio is lost in the compression process.

Ogg Vorbis

Newer, open source codec. Good quality and free to use, no license fee required. Also a Lossy codec, but considerably more efficient than MP3, especially at lower data rates.

AAC (Advanced Audio Codec)

Newer codec that is very efficient at medium bitrates. Proprietry and relatively expensive to license, so not as widely used as some others. Also a Lossy codec.

FLAC (Free Lossless Audio Codec)

Unlike the other three, FLAC is a Lossless codec, so enables streaming at full CD quality. Like Ogg Vorbis it’s Open Source and has no cost to license.

Bitrate

Bitrate is literally the amount of information (in bits) that you can stream in a second, usually measured in kbps (kilobits per second). More bits usually means better quality, but this also depends on the efficiency of the codec. For example, although 128 kbps MP3 is the same bitrate as 128 kbps Ogg Vorbis or 128 kbps AAC, the MP3 will sound significantly worse, since MP3 is a less efficient codec than the others.

What does this mean in practice?

People hear things in different ways, so it’s hard to draw definite conclusions, but audio engineers who have studied the subject generally agree on the following:

• 96 / 128 kbps is the lowest reasonable bitrate for listening to music — music streamed below that rate is perceived by most people as “annoying”. This bitrate can be used for limited data plan mobile connections, but it should really be considered an absolute minimum for music streaming. See Appendix 1 for more details on this.
• 160 / 192 kbps is acceptable for most people, with only a very small difference in perceived quality over higher bitrates. It should be considered a minimum quality bitrate for decent audio listening. It’s a good compromise for mobile connections where bandwidth might be limited.
• To get something that sounds close to CD quality you need a bitrate of at least 256 kbps for AAC or 320 kbps for MP3 and Ogg Vorbis. Tests have shown that most people can’t tell the difference between these and CDs.
• FLAC runs at a variable rate of around 1000 kbps. That’s about 3-4 times the data of the others, so you can see there is a big difference. However unlike the others it is exactly the same quality as CD. It’s perfect for people on home networks who really care about audio quality, but the extra bandwidth required means it’s not ideal for those on a mobile connection.

Streaming services and what they use

There is a lot of misinformation out there about what services use what format, but as best I can ascertain the following information is accurate:

Apple Music

Apple Music uses the high efficiency AAC format. As far as I can tell, it has only 1 quality option.

• 256 kbps: all streams

Deezer

Deezer uses 3 quality ratings, but it’s highest (HiFi) quality is only available bundled with selected hardware systems (e.g. Sonos), or via their beta desktop app on Windows and Mac OS.

• 128kbps (MP3): Free account
• 320 kbps (MP3): Premium+ account
• ~1000 kbps (FLAC): HiFi account

Deezer

Spotify

Spotify uses several quality ratings for streaming, either the medium efficiency Ogg Vorbis format for their apps, or the high efficiency AAC format for their web player. Here’s the official Spotify rundown of what they use:

Spotify have promised to release a lossless HiFi verison as well but at the time of writing (early 2022) this has still not appeared.

Tidal

Tidal is one of the few services to offer full CD quality streams in FLAC format, but it offers other quality options in AAC as well. It also offers a limited selection of so-called “hi-res” audio in MQA (Master Quality Authenticated) format. See Appendix 2 for more information on hi-res audio.

• 96 kbps (AAC+): Normal quality:
• 320 kbps (AAC): High quality:
• ~1000 kbps (FLAC): HiFi quality
• ~1400 kbps (MQA): Tidal Masters

How it works in practice

Free service

If you don’t want to pay for streaming the best options as regards sound quality are:

1. Spotify
2. Deezer

Spotify offers 160 kbps Ogg Vorbis (when set to High), which creates a file that is fairly decent quality, with only a small perceptible listening difference compared to higher quality systems.

Deezer offers only 128 kbps MP3; that’s not only a significantly lower bitrate than Spotify, it’s also a much worse codec. Tests show that many people will find this difference to be “perceptible but not annoying”. However their free service does offer some features that other free services do not.

Paid service

Real hi-fi buffs will go for Tidal’s Hifi or Deezer HiFi settings since they are the only ones with real CD quality offerings. Deezer’s HiFi service has a huge catalogue of CD-quality music (36 million tracks!), and is available to all subscribers. So Deezer definitely wins here.

Below that it gets a little more difficult. We have a second tier of “close to CD quality” from the following services, in order of quality:

1. Tidal High Quality (320 kbps AAC)
2. Spotify Very High quality (320 kbps Ogg Vorbis)
3. Deezer Premium+ (320 kbps MP3) / Apple Music (256 kbps AAC)

Realistically most people won’t hear a huge difference between any of these second tier services. Tidal and Spotify may sound a little bit better than the other options, and all else being equal, that might be enough to swing it in favour of one of those services.

Mobile

On mobile you always have to consider the trade-off between audio quality and data use. Streaming lossless FLAC files via a standard 4G phone connection is going to use a giant chunk of data and it’s not realistically feasible. So mobile streaming is all about the compromise between quality and data use — the higher the quality, the more of your data plan you are going to use.

Having said that, that doesn’t mean that mobile streaming has to be bad. For example Apple Music keeps its stream at near-CD quality 256 kbps AAC which is hard to beat as regards quality vs. limited streaming bandwidth. It’s still a fair amount of data, but it will sound pretty good.

Some of the other services are barely usable, with Deezer’s 128 kbps MP3 and Tidal’s base offering of 96 kbps AAC being pretty much a minimum when it comes to quality. They will keep your data use very low though, so that’s something. Frankly they would keep my data use to zero because they sound terrible.

Spotify offers a very good compromise solution with their 160 kbps Ogg Vorbis High quality option that does not sound bad at all, while still keeping data use relatively low compared to Apple Music. So the winners here are:

1. Apple Music
2. Spotify

Headphones & Bluetooth Audio

It’s important to remember that if you don’t have a decent set of headphones / speakers, it’s really not going to matter what service you listen to, because even a high quality stream will sound terrible on low quality headphones.

Another complicating factor is Bluetooth. Bluetooth is not capable of transmitting Lossless audio, period. Instead it uses various codecs of its own (mostly different to streaming audio codecs) to recompress the audio transmission between your device and your headphones. This can mean a significant loss in quality compared to the original audio. So if getting the best audio quality is an important issue to you, probably the best thing you can do is junk your Bluetooth headphones and get a decent wired pair instead.

There are a multitude of different headphones available, of vastly differing quality. In general, it’s best to buy a good wired pair from a reputable audio manufacturer like AKG, Sennheiser, Beyer Dynamic, or Sony — you’ll be happy you did. Avoid “cool” brands like Beats and Skull Candy, that stuff is generally overpriced crap — some of those headphones are built in Chinese sweatshops for less than $20 and retail for over $200.

Conclusion

It could be (and has been) argued that there aren’t enough people really interested in sound quality to make a significant difference in their choice of streaming service. However in a crowded space, sound quality is a differentiator that is increasingly important to a lot of people, especially tastemakers, artists, reviewers etc. It’s particularly important at the low end, especially on mobile, where low quality sound becomes very noticeable.

From the above, my feeling is that although Deezer and Tidal currently have the absolute best quality going, Spotify seem to have nailed the sweet spots better than anyone else. They offer a free tier that is listenable (though only just in Spotify’s case), and offer a paid tier and a mobile offering that sound very decent. When they finally release their promised lossless HiFi service they will lead the pack in every tier.

The biggest disappointment here is Deezer’s free tier: 128 kbps MP3 may have been “standard” 10 years ago, but in the modern streaming ecosystem it’s noticeably inferior to its competitors.

As I said at the beginning of this article, I’m focusing purely on sound quality here, not other factors. I’ve personally tried all of these services and none of them are the “perfect solution” to my own needs, never mind anyone else’s. At the end of the day I would recommend people try them out for themselves and see what they like — and remember that if a service is streaming at 320 kbps (as most of them are on home systems) the average listener will be getting pretty damn close to the best quality their speakers and headphones can handle anyway.

Appendix 1: Quality perception of lower bitrate audio

ODG (Objective Difference Grade) measurement shows that MP3 format audio needs to be at a bitrate of at least 160 kbps for most people not to notice a quality difference.

In tests, 128 kpbs MP3 has a ODG score of -1.08 (“perceptible, and annoying”), and Ogg Vorbis has -0.34 (“perceptible, but not annoying”).

At 160 kbps, MP3 improves to -0.47 (“perceptible, but not annoying”) and Ogg Vorbis to -0.20 (“almost imperceptible”), showing Spotify’s choice of 160 kbps Ogg Vorbis to be a wise one.

It is visible that all codecs act similarly at higher (>160 kbps) bitrates… On the lower bitrates (<160 kbps) on the other hand, we can see different behavior of all four codecs, especially in the most interesting 128 kbps. The best is OGG Vorbis, very similar qualities have AAC and MP3, MP2 has the lowest quality at this bitrate. From these results we can conclude that it is very important to pick the right codec at lower bitrates while it is not so important on higher bitrates in the terms of audio quality.

https://pdfs.semanticscholar.org/f533/5e6ebafb835d11a3ad4f713bbfd1eab4df55.pdf

In other subjective listening tests of 128 kbps MP3 vs lower bitrate 96 kbps AAC and Ogg Vorbis, we see that for many listeners 128 kbps MP3 is perceived to be about the same quality as 96 kbps Ogg Vorbis, of “perceptible, and annoying”.

http://listening-test.coresv.net/results.htm

Appendix 2: “Hi-resolution” audio

Several people have asked me about “hi-resolution audio”, and why I don’t deal with it here.

Despite a great deal of marketing and commentary to the contrary (especially by “audiophile” magazines and reviewers), it is effectively impossible for the vast majority of people to tell the difference between CD quality stereo and so-called “hi-resolution” formats. So in a paper meant for a general audience I don’t think it’s worth delving deeply into the subject. However for those who are interested in the technical details, here’s a link to the classic Audio Engineering Society paper on the subject that debunks the myth:

AES E-Library " Audibility of a CD-Standard A/DA/A Loop Inserted into High-Resolution Audio…

Also note that the much vaunted MQA “hi-res” format used by Tidal is not really high resolution at all — it’s actually a lossy codec and as such in most cases it will be significantly worse than the open source lossless FLAC format. MQA’s claims of effectiveness have been widely debunked and it’s now considered by many professional audio engineers (myself included) to be little more than a scam.