By
robertG
Wed Apr 19, 2006 10:06 pm
found the answer to question 2 : but looking for advice re the s950 still from those with hands on experience, also what floppys do s950's take DD or HD?
I shall share Re-sampling
The MPC2000XL is able to re-sample at different bit-depths, sample rates or a combination both. Available bit depths are 8, 12 and 16-bit and sample rates are anything from 65 kHz to 4kHz. The XL's re-sampling is a fairly slow process since the V53 processor does not have a floating point processor so the mathematical operations are performed in software. When re-sampling, the XL creates a memory space for the new sample and processes the samples data as it places them in the new location. For the below tests I used low, med and high pitched samples that were sampled from the output of a Lexicon MPX1 using a large room reverb. I tend to notice changes reverb trails more than anything else so I like to use them when I am testing gear.
For the XL, changing bit depth does not use less memory or disk space, but just zeros the last 4 bits of the 16-bit sample. I imagine the MPC is calculating this mathematically (multiplies the 16-bit word by the ratio of the change in bit depth: sample * 4096 / 65536 for 16->12bit). I could simulate the same operation on software and create MPC60 format *.snd files which uses a slightly more compact method of storing the same amount of 12-bit data (uses 3 bytes to store 2 samples). This would allow more data to fit on a floppy, which the MPC2000XL can still read. Though, it would not re-save in the same format. I might add this to the MPCEditor in the future.
For changing bit depth, I compared reducing from 16-bit to 12-bit for several samples at LOW, MED and HIGH quality. For this operation, there was no difference between the data when using the different settings of quality. However, when MED or HIGH was used the sample data itself was pushed back a few microseconds and the last few microseconds was truncated so the sample retained the same length. For most operations this small shift in the data would be insignificant. Regardless, since the data itself was the same, there is no reason to use MED or HIGH quality for this operation and using MED or HIGH even took more time. For these reasons, I suggest always using LOW quality when just reducing bit depth.
The MPC2000XL resampling is often compared to the older MPC60 and is often the subject of a lot of debate. The XL always samples at 16-bit and the samples can be re-sampled at a later time and the MPC60 also samples at 16-bit since it too has 16 bit converters, but the MPC60 just drops the last 4 bits during this process. The result is the same data (assuming there is little difference between the ADC's).
Changing sample rate is a bit more of a complicated process. Like changing bit depth, changing sample rate has to throw data away. I am not terribly familiar with the mathematical methods to re-sample data (I believe there are three commonly used), but I would risk a guess that the three quality selections might correspond to these three methods. This might also be indicated by the fact that I did not detect a significant difference in an actual quality between the new samples for the examples I was using. I did notice the changes in the actual resampled data, but I cannot offer additional comments on this operation except that selecting a quality for this operation is going to be a matter of preference. Once I am able to manually perform re-sampling myself, I will be able to compare my output to the MPC's and offer additional insight.
When changing sample rate, there is actually less data in the sample. While the MPC still plays back the sample 44.1kHz, it compensates for this by changing the tuning of the sample. For example, if you re-sample from 44.1 to 22.05 kHz, the MPC will adjust the new sample's tune to -120. Halving the sample rate will raise the sample pitch one octave, so the MPC tunes to the sample down one octave (-120). You can see the new tune under TRIM/PARAMS. Note that this is a tune for the sample, not for the pad, which is a separate setting. All tuning changes made for the pad will be based on the tune for the sample. The `automatic` adjustment for the sample's tuning is limited to one octave (-/+ 120) so if you want to re-sample at less than 22050 kHz you will need to make further tuning adjustments for the sample under it's pad settings.
