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(Pocket-lint) - If you've ever listened to a CD recording of your favourite album next to a stream from, for example, Spotify, you may well notice a difference. That's because the quality of the recording on the CD is significantly greater than the compressed stream, particularly when you're listening to your stream over Bluetooth. 

High-resolution audio aims to give you sound quality that matches or betters CD quality in the convenient package of streaming from your phone or as a digital download on a dedicated player.

Tidal is one of the most well-known services to offer hi-res audio. However, other providers have upped their game with many offering higher quality audio. 

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Why is music compressed?

Before we get into high-resolution audio, let's start by discussing compression.

It's no secret that the music industry went from physical formats (like vinyl) to digital formats (like MP3), but most people don't realise sound quality was impacted during that transition due to a demand to make music files smaller. Simply put: they're easier (and cheaper) to distribute to consumers who download or stream.

In order to make a music file smaller, it must go through either lossless or lossy compression. With lossless compression, all original data from the music file can be restored after it is uncompressed, whereas lossy compression permanently reduces the music file by cutting out some data during the encoding process and makes it much smaller.

Whenever any data is lost or removed from a music file during compression, the resolution is negatively impacted.

It was storage and tranmission demands that prompted audio compression in digital forms, and with the cost of storage dropping, and the wider availability of faster mobile and broadband data, this has prompted a move to higher quality music.

What is high-resolution audio?

There's no universal standard for high-resolution audio, which is also called hi-res audio or HRA, though The Digital Entertainment Group (in conjunction with the Consumer Electronics Association and The Recording Academy) has published the following formal definition for the term:

  • Hi-res audio is: "Lossless audio that is capable of reproducing the full range of sound from recordings that have been mastered from better than CD quality music sources."

Now, when music is digitised for downloading and streaming purposes, it is basically broken down into a series of audio snapshots (sort of like how several still frames make up a film), and our brains interpret all these snapshots together as continuous sound. The more snapshots taken, the more detail a digital music file will have.

We won't get too scientific - but just know that sampling rate is the number of snapshots taken per second when analogue sound waves are converted into digital. There's another aspect called bit depth. It's the amplitude of the waveform at each sample point (8-bit has 256 levels for each sample; 16-bit has up to 65,536 levels). That's the amount of data of these snapshots - the higher the sampling rate and bit depth, the more information is in that audio file.

So, to recap: hi-res audio refers to audio files that have a higher sampling rate and more bit depth than a CD - and it is generally best to have the highest combination possible. A CD has a sampling rate of 44.1kHz and a bit depth of 16-bit, while hi-res files have a sampling rate typically ranging between 96kHz and 192kHz at 16- or 24-bit.

Then you have bitrate, which is expressed as the amount of data per second in that audio file. This is often what you'll see expressed on streaming services. A CD would be 1411kbps, while some streaming might be as low as 96kbps.

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What are some hi-res file formats?

There are several high-resolution audio file formats available with incredible sampling rates and bit-depths. Two of the most popular types are FLAC (Free Lossless Audio Codec) and ALAC (Apple Lossless Audio Codec), both of which are compressed but claim to hold onto and recover every single bit of data from a file. 

Other file formats you may have seen that are considered hi-res include WAC, AIFF and DSD. More recently we've seen MQA (Master Quality Authenticated), which aims to deliver the original studio recording with 100 per cent of the data. 

Not all devices can decode these formats and not all services offer all formats, however. MQA's highest-profile supporter is Tidal, with a range of Master level tracks which use MQA to deliver the hi-res audio. Tidal also uses FLAC for its "normal" hi-res music. Apple Music lossless uses ALAC.

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Which services offer hi-res audio?

There are a range of services offer high resolution music, but here we're covering the big players for streaming.

Tidal is the name you'll be familiar with. There are two tiers of Tidal - HiFi, which is up to 1411kbps streaming and HiFi Plus which is anything from 2304-9216kbps. This refers to the bitrate offered, while you'll find various qualities within the Tidal platform, including those MQA Master files we mentioned earlier. These are only available to the HiFi Plus tier of subscibers, however, but both levels of Tidal are considered to be high-resolution. Prices are from $/£9.99 a month for HiFi or $/£19.99 a month for HiFi Plus.

Tidal is widely available on Android and iOS, desktop and supported by many high-end audio devices.

Qobuz is both a music streaming service and download store. It offers high-resolution audio up 16-bit/192kHz and costs $12.99 or £10.83 a month for Studio Premier. There's two tiers, with a £15 Studio Sublime tier also offering discounts on hi-res purchase to download.

Qobuz is widely supported with Android and iOS apps, and support on high-end audio devices.

Apple Music offers CD quality (16-bit/44.1kHz), Apple Music Lossless (24-bit/48kHz) and Hi-Res Lossless (up to 24-bit/192kHz). To listen to Hi-Res Lossless, you'll need to use an external DAC, i.e., it's not supported by Apple's own equipment. Apple Music starts at $/£9.99 a month.

Amazon Music HD was once a premium tier, but is now availalbe to all Amazon Music subscribers, will run up to 24-bit/192kHz, with a bitrate of 3730kbps. Prices start at $/£7.99 a month for Prime subscibers or $/£9.99 a month standalone. 

Spotify HiFi has been rumoured for some time, but currently isn't hi-res, using compressed AAC at up to 320kbps.

Do I need special equipment to listed to hi-res music?

In many cases, you can stream high-res music through a smartphone or computer - or connected media player - without anything else. But you do need to consider what's happening to your music. If that source is then being processed by your device, and perhaps then transmitted wirelessly, you could be processing away a lot of the quality.

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To get the best out of your high quality music, you need to know what the DAC - digital to analogue converter - is doing. There's a DAC in every computer and phone, but dedicated external DACs offers a better quality of processing, handling that data better to give you a better sound at the end of the process. This is more important for those chasing bit perfect music, so if you're thinking of subscibing to Tidal HiFi Plus for those MQA Master tracks, then you will want a compatible DAC to handle MQA. If you don't have such a DAC, the app will process those files itself, so you can still listen to them. 

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Even Apple Music Hi-Res Lossless will require a DAC to decode it. That's not something that Apple offers in its own devices - you'd need something to unpack and process the stream - like the Chord Mojo 2 for example.

While you don't necessarily need special headphones or speakers, the better the headphones and speakers you are using the better the experience will be. If you want to take advantage of the best quailty, you'll want headphones that can handle the delivery of higher quality music - with many now branded as HiRes too. Of course, at the top end, wired headphones are the way to go, to remove the additional processing involved with wireless transmission.

Writing by Maggie Tillman and Elyse Betters.
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