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33.
Synchronisation - Detailed Explanation
The Prodigy Editor can track the position and motion of external transports such as a Video Tape Recorder,
timecode-striped audio tape or a timecode generator. The disk recorder will play in time with the Master Timecode
source so that sound and picture coincide.
1.
Position
This is an absolute location reference to a sequence of pictures or audio. It is used to determine whether the disk
recorder is playing the right part of its project. For video, position reference is usually provided by 9-pin (Sony
protocol from an RS-422 port). For audio tapes LTC is normally used (though 9-pin is also a possibility).
2.
Motion
The motion of an external machine is a measure of its speed, and the disk recorder must move at the same speed to
remain in sync. This translates into producing the correct number of samples every second, which is called the
Master Clock rate. This can be locked to a Digital Word Clock, a video signal, an AES/EBU timimg signal, by the
internal crystal, a timecode source, or a Digital audio source which is being recorded.
If the Position Reference and Motion References are not the same, it is possible that they will drift apart over time.
This will be shown by a warning at the top of the Prodigy Editor video screen which indicates when an inconsistent
timecode frame was encountered.
How the Prodigy Editor Synchronises
The Prodigy Editor goes through a number of steps in achieving synchronisation in play mode:
1.
Read the position reference and start loading up the corresponding audio on all active
tracks.
2.
When ready, start playing, but with the outputs muted. Use variable speed to reduce the
error between the Prodigy Editor position and the Position Reference until it is very small.
3.
Switch to the Motion Reference for continued playback. The system is said to be "locked"
once this switch is made, and the audio is unmuted.
4.
Warn of error status if the sync error increases, if a reference signal is lost etc.
Digital Synchronisation Conflicts
When you are recording a digital source, it must be synchronised to the same Master Clock Reference as the
Prodigy Editor, or an overflow (too many samples) or underflow (too few samples) may occur at the input. This
causes a characteristic, periodic form of digital distortion called a "whisper". A whisper sounds like a brief, "glassy"
or "metallic buzz, and occurs about once every few seconds, depending how fast the samples are overflowing or
underflowing.
One way to guarantee digital synchronisation is to choose the digital input signal itself as the Master Clock
Reference for MFX. This is done in the Digi Menu by selecting INPUT under the Sync Soft Key or AUTO under the
Inp Sync Soft Key. This latter source of timing will only be used when a digital input is armed, with the normal
HOUSE sync used at other times.
A side effect is that the word clock rate of all digital outputs will also sync to the digital input, which may cause
digital synchronisation conflicts downstream from the MFX3. For example, trouble may occur if at the same time as
recording your digital input, your MFX3 outputs are feeding the inputs of a digital console. If you choose INPUT,
the digital console will be required to sample rate convert all of MFX3's outputs, or synchronise with its Master
Clock.
The best solution to digital sync conflicts is to drive all digital devices from a single, studio-wide word clock
signal. Then all devices will have identical word rates, so whispers never occur and digital interconnections may
made with ease. In this instance, MFX3's sync HOUSE option is always used.
Pull-up and Pull-down
The "normal" sample rates used by the digital audio industry are 32,000, 44,056. 44,100 and 48,000 samples per
second. The normal frame rates are 24, 25, 29.97 and 30. These last two may cause confusion when used together.
MFX3 can alter its sample rates far enough to accommodate the small speed change caused by going from 30 to
29.97 frames per second and back again. In other words, if you slow down the frame rate from 30 to 29.97, MFX3
can slow its internal sample rate to match. This is called Pull-up or Pull-down, depending in which direction you
are taking the speed.
When you choose a frame rate you are telling MFX3 what frame rate to "expect". To put it more scientifically, you
are telling it the frame rate at which the chosen sample rate will be accurately reproduced. For example, if you tell
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