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What is gapless playback?

  • Gapless playback – “the uninterrupted playback of consecutive audio tracks” – is a concept that’s well known to veteran music streamers. Newcomers not so much. Less than twenty-four hours after the following video went live on YouTube, numerous commenters seemed baffled by the concept of gapless playback. What is it? Why does it matter? Don’t songs on an album already have gaps?

    We’ve gone back to basics. But not far enough.

    Take Talking Heads’ Little Creatures. When we play the CD, a laser works its way across the CD’s surface, reading a single track of audio as it goes. The table of contents (ToC) file stored at the very beginning of the CD tells the CD player’s display to advance the track number at specific points; this gives us the illusion that we are listening to ten discrete tracks. The ToC’s virtual track markers also allow us to skip from one track to another – from, say, “And She Was” to “Walk It Down” or from “Road to Nowhere” to “The Lady Don’t Mind” – but in reality, we are moving forwards and backwards along a fifty-minute audio track.

    This changes when we rip the Little Creatures CD to a hard drive. The ripping software reads the CD’s single track but splits its contents into separate files according to the ToC. The ripping software will also compress those tracks according to user-specified settings and inject any metadata: Lame MP3 and FLAC are two of the most popular formats. After ripping the CD, we will have ten separate audio tracks on our hard drive – one for each song.

    If we then stream that album’s ten tracks across the network, the streaming protocol in play may splice tiny gaps of silence between the tracks. This is usually caused by a single-threaded process running on the streamer’s CPU that stops and starts as it finishes reading one track and starts reading the next. This is not gapless playback. It’s gapped.

    For an album like Little Creatures, it’s a non-issue. Talking Heads made the artistic decision that the songs on this album should stand separately, bookended by silence. This between track silence is in the recording and we hear it when we play the CD. A little extra silence spliced in between tracks by a network streamer will probably go unnoticed.

    This changes if we cut over to the 1982 album The Name Of This Band Is Talking Heads. For the majority of these live tracks, there is no intra-track silence. When we play the CD, “I Zimbra” is carried over to “Drugs” by audience noise; “The Great Curve” moves seamlessly into “Crosseyed and Painless”, again by audience noise. We hear no between-song silence because the artist intended there to be none. But if we play the ripped files with a network streamer only powerful enough to run a single-threaded process, it will splice tiny gaps of silence into those audience noise transitions.

    This is a serious irritant for those who see music listening as an immersive experience and not just a soundtrack for another task. The tiny gaps of silence inserted by less-capable network streamers jerk us momentarily out of any immersion. At best, they are a minor annoyance, at worst they are properly jarring. On a deeper level, they are not as the artist intended.

    It’s not just live albums that are disturbed by a network streamer’s lack of gapless playback. Classical pieces are also affected and it renders DJ mixes unlistenable. Seamless track transitions sit at the very heart of DJing. Any album where the artist has chosen to blend one song seamlessly with the next will see its artistic intent compromised.

    The most common form of streaming – but without a network – is Bluetooth. The audio travels digitally, from smartphone to receiver, as a single uninterrupted stream. If we hear gaps between tracks, it’s because of the smartphone app handling playback, not the Bluetooth stream that carries it.

    Also providing a continuous stream of audio data from smartphone to streamer is Apple’s AirPlay. With Airplay-capable network streamer and smartphone connected to a home network (router), whatever audio is emitted by the smartphone will be relayed directly to the streamer. AirPlay’s advantage over Bluetooth is that it doesn’t’ compress audio data but it’s not without its own shortcomings: a) only iPhones, iPads, Mac and Macbooks can act as source devices and b) AirPlay is more power-hungry than Bluetooth, running down smartphone batteries at a much faster rate. This second reason is why I never use AirPlay on my iPhone.

    More egalitarian in its approach to operating systems is Spotify whose Connect wizardry runs inside any Spotify app used by Premium subscribers: it allows us to forward Spotify’s cloud stream to a Spotify Connectable streamer running in our hi-fi rack. Instead of the smartphone pulling down the stream from the Spotify server, the network streamer goes Cloud direct. Turn the phone off and the music keeps playing. And since 2012, Spotify streams gaplessly. It splices no silence between tracks.

    The Google Chromecast Audio puck works in a similar way – it can take over any stream kicked off by a Chromecast-enabled app: Tidal, Qobuz, Deezer, SoundCloud, Mixcloud and Google Play Music and Spotify (to name seven) all make the cut. The big disadvantage for would be immersive listeners is that the Chromecast puck doesn’t offer gapless playback; the stream stops and starts between tracks. For SoundCloud and Mixcloud’s DJ mixes that play inside a single track, it’s a non-issue.

    Google has discontinued the Chromecast Audio but their Chromecast Built-in program lives on to remain one of the few ways that manufacturers allow people (like me and probably you) to spin SoundCloud content on the big rig.

    Whatabouters pointing to DTS Play-Fi are reminded that the audio stream travels through the smartphone on its way to the Play-Fi-enabled network streamer — in other words, it’s like an AirPlay that doesn’t do gapless. Like Chromecast, its audio stream is interrupted as it moves from track to another. My advice: avoid.

    These gapped interruptions continue to stalk UPnP streaming implementations: some are gapless, some aren’t. Some can be made gapless with apps like Glider (iOS only) and mConnect Lite. And you won’t know for sure until you get the streamer home and try it for yourself. It’s because of these gapless inconsistencies that I avoid UPnP whenever possible. One workaround is Bubble UPnP (Android only). My experience with it is limited but its party trick is that Bubble UPnP (effectively) turns the Chromecast Audio puck into a gapless UPnP renderer (streamer) by sending it a single uninterrupted stream.

    Roon plays gaplessly to all Roon Ready streamers, AirPlay devices, Sonos devices and Squeezeboxen. And because it sends out digital audio on a single uninterrupted stream, it streams gaplessly to the Chromecast Audio too. Even without Roon intervention, Sonos devices and Logitech Squeezeboxen (and their emulators) stream audio gaplessly.

    Gapless playback is fundamental to our enjoyment of music. Without it, I avoid albums I truly love like Global Communication’s 76:14 and The The’s Mind Bomb. I know the gaps artificially inserted by the playback system will ruin the immersion. Store-bought CDs play gaplessly. Vinyl records play gaplessly. In 2020, network streamers that don’t remain unacceptable.

    Written by John

    John currently lives in Berlin where he creates videos and podcasts for Darko.Audio. He has previously contributed to 6moons, TONEAudio, AudioStream and Stereophile.

    Darko.Audio is a member of EISA.

    Follow John on YouTube or Instagram

    PS Audio goes inter-Stellar (again)

    Podcast: Who the hell is Chris Whitten?