Streaming Audio Quality 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 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]
There are two main variables that affect the quality of the sound you hear on a streaming music service:
- The codec used to compress the audio
- The bitrate, or amount of compression applied to the audio
Let’s examine these two in turn.
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. The main ones used today by streaming services are (in order of perceived quality):
- Ogg Vorbis
Oldest in general use, very widely supported but not great quality, especially at low bitrates. License to stream with it costs money, but not too expensive. It’s a Lossy codec, which means that some of the original CD audio is lost in the process.
Very new, open source codec. Fairly decent quality and free to use, no license required. Also a Lossy codec.
AAC (Advanced Audio Codec)
Newer codec that is very efficient at low bitrates. Expensive to license, only used by Apple and Tidal for music. Also a Lossy codec.
FLAC (Free Lossless Audio Codec)
As the name suggests, this compresses the file without any loss of quality, unlike the other three above. It basically allows streaming at full CD quality, so unlike the other three it’s a Lossless codec. Like Ogg Vorbis it’s Open Source and free to license.
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 is ok for limited 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 most purposes, and perfect 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 AAC or 320 kbps MP3 or 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, so you can see there is a big difference in the amount of data transferred compared to any of the others. 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 good 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:
Deezer uses 3 quality ratings, but it’s highest (Elite) quality is only available bundled with selected hardware systems (e.g. Sonos).
- 128kbps (MP3): Free account
- 320 kbps (MP3): Premium+ account
- ~1000 kbps (FLAC): Elite account
Deezer is the No. 1 site for listening to music on demand. Discover more than 40 million tracks, create your own…www.deezer.com
Spotify uses 3 quality ratings for streaming, all in the medium efficiency Ogg Vorbis format.
- 96 kbps: Normal quality on mobile.
- 160 kbps: Desktop and web player Standard quality. High quality on mobile.
- 320 kbps (only available to Premium subscribers): Desktop High quality. Extreme quality on mobile.
Apple Music uses the high efficiency AAC format. As far as I can tell, it has only 1 quality option.
- 256 kbps: all streams
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
Google Play uses only MP3, but has a clever system which dynamically adjusts your bitrate depending on how good your Internet connection is. On a good connection, like a home wi-fi, you’ll get 320 kbps MP3, but on slower connections, like a mobile phone, you will get a lower rate stream.
- Up to 320 kbps MP3 (dynamic)
How it works in practice
If you don’t want to pay for streaming the best options as regards sound quality are:
- Google Play
(Note: this is just based on sound quality, it does not take into account other factors such as User Interface, catalogue available etc.)
Google Play offers near CD quality even on their free tier, which is hard to beat — however their free service is pretty limited.
Spotify offers 160 kbps Ogg Vorbis, which creates a file that is fairly decent quality, with only a small perceptible listening difference 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 many features that other free services do not.
Real hi-fi buffs will go for Tidal’s Hifi or Deezer Hi-Fi settings since they are the only ones with real CD quality offerings. Deezer’s Hi-Fi service has an huge catalogue of CD-quality music (36 million tracks!)and as of November 2017 is available to all subscribers — it was previously limited to selected Sonos hardware owners only. 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:
- Tidal High Quality (320 kbps AAC)
- Spotify Premium (320 kbps Ogg Vorbis)
- Deezer Premium+ (320 kbps MP3)/ Google Play (320 kbps MP3) / Apple Music (256 kbps AAC)
Realistically most people won’t hear a huge difference between any of these second tier services. For some people, Tidal and Spotify will sound a little bit better, and all else being equal, that might be enough to swing them in favour of one of those services. However they are more likely to be swayed by other factors.
On mobile the big winners out of the gate are Apple Music and Google Play. Apple keeps its stream at near-CD quality 256 kbps AAC, and Google Play adjusts dynamically to always give the best sound possible, up to 320 kbps MP3.
All of the other services are barely usable, with Deezer’s 128 kbps MP3 and Tidal and Spotify’s offerings of 96 kbps AAC. However, Spotify also offers a 160 kbps Ogg Vorbis High quality option on mobile that is really not bad at all, and an 320 kbps Ogg Vorbis Extreme quality that is excellent. So the winners here are:
- Apple Music
- Google Play
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 definitely have the best quality going, Spotify and Google Play seem to have nailed the sweet spots better than anyone else. Both offer a free tier that is listenable (though only just in Spotify’s case), and both offer a paid tier and a mobile offering that sound very decent — with Spotify again managing to beat out the rest of its main competitors.
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.
And note that if you don’t have a decent set of headphones, it’s really not going to matter what service you listen to, they will all sound pretty terrible. Go get yourself some decent phones from a reputable audio manufacturer like AKG, Beyer, Sony, or Blue and you’ll be happy you did.
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.”
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”.
Appendix 2: “Hi-resolution” audio
Despite a great deal of marketing and commentary to the contrary (especially by so-called “audiophiles”), it is effectively impossible for the great majority of people to tell the difference between CD quality stereo and “hi res” formats. Basically Hi-Res audio simply isn’t worth bothering with.
This is the classic Audio Engineering Society paper on the subject: