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Limitations of white noise..

Posted by flood2 (A) on October 23, 2014 at 16:03:04

In Reply to: RE: Interesting! Thanks!... posted by John Elison on October 23, 2014 at 10:29:06:

This is an interesting point you raise. As you noted, the white noise response is not a true representation of the actual transducer response. I should actually be establishing the transfer function with respect to the input signal. However,the difficulty is in getting an accurate transfer function for the RIAA stage and also knowing for certain that the cutting process is truly uniform in order to know for sure that I have the actual transducer response. The other limitation is in the duration of the test tone to have a sufficiently large number of occurrences for each frequency component. Given these limitations, I use the white noise response a little differently - I have a chosen reference response and map differences of other cartridges back to the reference.
I do also have the CBS STR-100 test disc, but haven't been motivated to use this as my present method seems to give me the result that I'm after which is a mapping relationship.

The observation you made regarding the attenuation characteristic mainly applies below 1kHz and is dependent on the number of points chosen for the FFT as well as LF resonances due to arm/plinth and cantilever suspension interactions. I admit that I haven't been particularly consistent in the number of points chosen for my screenshots. Beyond 1kHz, the response is reasonably representative of the transducer response - I attach a trace for my AT-OC9 which I think you will agree is essentially uniform above 1kHz. This is my reference response for mid to HF uniformity - although I use iZotope RX for my main processing, like you, I use Soundforge to zoom in and get more accurate amplitude information to plot out relative amplitude. I then use the relative differences to digitally equalise my recordings (FIR to achieve linear phase) from all other (MM) cartridges to match the OC9 response. Generally I only need to apply EQ above 8kHz and I have been very pleased with the accuracy of the results which successfully minimise the tonal differences due to frequency response variations. Of course detail retrieval etc is still highly dependent on stylus profile/condition and arm matching, so one can still "hear" a better design. I still believe that the dominant effect on sound quality is related to stylus profile and alignment in the cantilever (laterally as well as SRA) combined with cantilever materials and the accuracy of the tracking angle more than the transducer principle. I guess that's pretty much a cartridge as a whole isn't it? ;)

Incidentally, I note that you mention your new recording capability with DSD - what is your new ADC? I'm still using a Benchmark ADC1-USB which is limited to 24/96 through USB. However, I have actually settled on 24/88.2 for two reasons; The first was technical, I eliminated problems with dropped samples and glitches (even using ASIO and a fast SSD), the second was a subjective improvement in SRC when down converting to 16/44.1 for creating CDs that my mother could enjoy. Good as iZotope's SRC is, I still feel that integer downsampling is preferable subjectively.


Regards Anthony

"Beauty is Truth, Truth Beauty.." Keats

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