Half Speed Vinyl Mastering is a technique that refers to the speed (RPM) that the cutting lathe turntable rotates, and the playback speed (both the pitch and time) of the audio to be cut.
Because both the audio and the turntable are running at half speed, when the finished disc is played back at normal speed the audio is also returned to the correct speed.
Vinyl mastering at half speed offers some potentially very good sonic benefits for a number of reasons;
- At half speed the cutting head amplifiers use only one fourth as much power, meaning there is four times the amount of spare capacity (head room) within the amplifier system, improving the ability of the amplifiers to respond to sudden demands for power. This results in greater dynamic response and better separation of individual instruments within the stereo image.
Additionally every event within the recording is happening twice as slowly as it does at normal speed, meaning the transient (attack) information is only half-as-difficult for the cutting amps to reproduce as it is at normal speed.
- The Neumann SX 74 Cutterhead that we use at Alchemy is the most technologically advanced head that Neumann ever designed, which uses feedback to balance its equalisation response. This feedback goes unstable at around 17kHz (leading to slight irregularities in the frequency response at this point), although the frequency response regains its evenness at around 19kHz, from which point it slowly tails off towards 30kHz and beyond.
Cutting at half speed means that information that is within the source recording at around the 17kHz area is actually recorded at 8.5kHz, and so is a long way from the point where feedback runs out. This results in a more accurate and more open high frequency response in the half speed vinyl recording when compared with a normal speed vinyl recording.
- As the vinyl master lacquer is rotating at half the normal speed, the cutting stylus has in effect twice the amount of time in which to move in order to accurately cut the groove, and the physical inertia that limits this process is reduced by half. This means the groove that is cut has the potential to be a more accurate representation of the source waveform, and contain greater detail. The high frequency roll off that occurs naturally on vinyl as the groove gets closer to the centre of the disc is greatly reduced, meaning the sound retains more high frequency even at the very inner diameters.
- At Alchemy we cut variable pitch, meaning two signals are sent to the lathe, one in advance of the other. This look-ahead signal gives the pitch control computer contained in the lathe the ability to know what is coming next, and therefore vary the amount of land between the grooves accordingly to make the best use of space on the disc. At half speed the pitch control computer works even more efficiently enabling better packing of the grooves.
This can result in louder cuts and means where required we are better able to keep the audio recording away from the centre of the vinyl disc where the playback frequency response is poorest (due to the limitations of the vinyl format).
- Audio recorded at half speed will replay at a louder level, in our experience it is usually by 3dB. This benefit coupled with the performance increase in the pitch control computer means we are usually able to cut longer sides, or cut louder than normal speed, or both.
Cutting louder means the signal to noise ratio of the finished record is improved, as we are able to raise the loudness of the recorded audio signal further away from the noise floor of the record.
The difference in sound between cutting at half speed and normal speed can be quite marked on the right program material – the bottom end extends deeper, is more rounded and the punch of percussive elements such as kick drums and low toms is faster and has more impact.
The low mids are richer and fuller, but without the overall sound losing clarity or becoming muddy. The high frequencies are smoother and the stereo image is vastly improved, seeming more 3 dimensional and more stable. The sound is sharper, more direct, and more engaging.
Whilst the many benefits of half speed mastering will improve most recordings it is not suitable for all program material, and the audio source material needs to be assessed (and where necessary tests need to be carried out) before it is decided that a half speed master is the appropriate technique to use.
Because we halve the speed and therefore the pitch of the source material during the cutting process any de-essing that is required cannot be done in real time (on the fly), unlike when cutting at normal speed. This means that overly bright and sibilant source material may not be as suitable for half speed cutting or may require significantly more work to prepare for a half speed cut.