What Master Tempo / Key Lock is and what it does to the sound.

What Master Tempo / Key Lock is and what it does to the sound

What it actually does

rekordbox describes MASTER TEMPO unambiguously: press [MT] and adjust tempo with [+]/[-], and the track’s pitch does not change even when playback speed changes. The same idea appears in three places in the manual: the function panel, the platter and the sampler deck, where it explains that a loop starts without altering pitch even when played at a different tempo from the original track.

Traktor approaches it from the other side: locking a track’s key lets you change its tempo without affecting its key. The button is literally called Key Lock (Preserve Pitch), described as locking the track’s current key.

To understand it fully, consider what happens without the feature. The rekordbox manual documents this: with master tempo off, the key display also changes if you modify tempo, for example through Beat Sync. This is classic vinyl behaviour: speed up and the music rises in pitch.

That gives the cluster’s mental rule:

• Without Master Tempo: tempo and pitch move together. Faster means higher.
• With Master Tempo: pitch stays where it was, and only tempo changes.

What each program calls it

The names are not equivalent, which is the first source of confusion.

In rekordbox, the name is MASTER TEMPO and the control is the [MT] button. A verified finding from this cluster’s research: “Key Lock” does not appear in the rekordbox 7.2.18 manual, based on a full-text search of its 266 pages with zero matches. Someone telling you to enable Key Lock in rekordbox is using a name that manual does not use.

rekordbox does have key functions, but they are separate features: Key Sync and Semitone Up/Down, described beside the panel’s synchronisation controls rather than within Master Tempo.

In Traktor Pro 4, the official name is Key Lock. The manual also documents that the Key knob operates only while Key Lock is on: it then controls a playing track’s key without affecting tempo. Traktor also allows semitone transposition of the original key with Key Lock enabled, and provides independent Key Lock for each deck and each Remix Deck slot.

Scope warning: everything verified here comes from the rekordbox 7.2.18 and Traktor Pro 4 manuals. This cluster has no verified Serato or CDJ data, so neither naming nor ranges should be extrapolated.

The cost: audio is reprocessed

This fact makes Master Tempo a decision rather than a switch to leave on permanently.

The rekordbox documentation includes a warning twice, once beside the function-panel description of Master Tempo and once beside the platter description: “The sound quality may change because the sound is digitally reprocessed”. The manufacturer is warning that maintaining pitch while changing speed is not neutral: digital audio processing takes place.

Traktor documents two additional, specific costs:

• Artefacts when crossing tempo zones. On variable-tempo tracks, the manual recommends enabling Key Lock on synchronised decks precisely to avoid audible pitch-shifting artefacts when crossing between tempo zones.
• CPU load. The manual mentions Key Lock as a demanding task when explaining multicore support: enabling it on every deck is an example of high load that the setting helps reduce.

None of this says it sounds bad. It says something more useful and honest: the sound changes and the system works harder. How noticeable that is depends on the material, how far you stretch tempo and the specific feature; evaluating it is the subject of the cluster’s next guide.

Documented range limits

Separate two easily confused things.

In rekordbox 7.2.18, no percentage range is documented at which Master Tempo switches off. The only tempo-related ranges the manual gives are the platter’s speed ranges, ±6, ±10, ±16 and WIDE, with stated precision of 0.02 %, 0.04 % and 0.5 %. Those limit the tempo control, rather than Master Tempo. Confusing them is a common reading error.

Traktor has an explicit limit belonging to a specific mode. Its manual describes Key Lock Modes as the range in which Key Lock operates when enabled. In Scratch mode, Key Lock switches off when playback speed drops below −30 % or exceeds +50 %, with the stated aim of achieving more natural scratch sound with timecode or jog wheels.

The only verified numerical limit therefore belongs to Traktor and is tied to a usage mode rather than the feature in general.

When enabling it is worthwhile

A practical heuristic rather than a written rule. For a typical mixing repertoire, most DJs keep Master Tempo or Key Lock on because it lets them align tempo without dragging key along, which keeps harmonic mixing meaningful. There is no official rule requiring it to remain on.

Two useful decision criteria:

• If key matters to you, enable it: aligning tempo will not displace pitch and break a prepared harmonic mix.
• If you will move tempo very little, there is less processing by definition and less risk of noticing reprocessing. This is common-sense reasoning rather than manual data.

A practical warning: if reprocessing raises doubts, check the result before the set, using the same track and tempo range you will play. Live performance is not the time to audit it.

Common mistakes

“Master Tempo and Key Lock are the same button with two names.” They do the same job, but each name belongs to its product. “Key Lock” is absent from the rekordbox manual; its name is MASTER TEMPO.

“Master Tempo does not touch the sound.” The rekordbox manual says the opposite: quality can change because audio is digitally reprocessed. Not noticing it does not mean it is not happening.

“rekordbox Master Tempo cuts out beyond a certain percentage.” That is not documented in manual 7.2.18. The −30 %/+50 % limit belongs to Traktor’s Scratch mode.

“Since the platter ranges are ±6 and ±16, Master Tempo works the same way.” They are different things: ranges limit how far you can move tempo; Master Tempo determines whether pitch follows that movement.

“The Key knob does nothing with Key Lock on.” In Traktor, it is the other way round: with Key Lock enabled, the knob controls key without changing tempo. Without Key Lock, it behaves differently.

“If it sounds strange, the track is wrong.” Before blaming the track, disable Master Tempo and repeat the tempo movement. If the artefact disappears, you know where it came from.

To decide when stretching becomes more noticeable and when switching it off is worthwhile, see When Master Tempo makes the sound worse.