Some Tips for getting low Difference Track
levels with
Audio DiffMaker
Note: Please download the current version (1.25) for now and use it to listen to the examples or to make your own examples with sample-synched systems. But check back in a week or two, as a new Version 2 should be ready then for free download! Version 2 adds capability to compensate for sample rate drift, an effect that can impair achievement of quiet difference results with otherwise identical tracks. Files you record with version 1.25 will still be compatible with (and can even be reprocessed by) version 2. -- thanks! -bw
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When using DiffMaker, you should be able to process two recordings made under identical circumstances to obtain an essentially silent difference track. If there are no differences in the setups, the Difference should be silent. You should probably run a "Dummy Test" like this for each setup to make sure it is capable of reliably detecting any real differences.
Some users have had trouble obtaining such a result in Dummy tests.
The most likely reason for this is clock frequency drift in digital recording and playback equipment. If you use a CD player, for example, as the source and a sound card to record, their clock frequencies will drift over time a very tiny amount, and not usually at the same rate and direction. In the time between when two recordings are made, the converter clock(s) may drift apart enough to make the two recordings slightly different in speed, so that samples cannot precisely line up and subtract correctly.
Even very tiny amounts of clock mismatch (less 0.0001%) is sufficient to badly spoil a Difference track! A symptom of this is that the Difference Track will have significant signal levels at beginning and end, with a distinct quiet spot near the middle. That happens because DiffMaker lines the tracks up in time as best it can, which gives a good match at the midpoint and poorer matches to the ends. Really bad clock mismatches can give a "phlanging" sound effect as well.
To fix this problem try these methods:
1) Set up the test so that all digital devices synch to a common clock. This is the very best technique. If DiffMaker is used to provide the Source signal with the same soundcard that is being used to record with, that will automatically take care of it (with most cards).
If you can't do that (for instance, if you want to use a CD as the music source), try to synch your soundcard to the SPDIF output of the CD player. Some soundcards are able to lock their clock to an external digital signal and use it for the analog recording. The ESI Jul@ sound card is particularly good for this.
2). Make the recordings as close together in time as possible. This will give the clocks less chance to drift very far apart. Have the equipment change ready to implement in advance so that minimal time passes between making the recordings.
3). Warm up the equipment for several hours before doing the test, so their clocks can stabilize better.
4) Keep the room temperature carefully controlled. Clocks also drift from temperature changes.