QRange Distortion

Version 1.1.4 of QRange added an option to the settings menu called “Process Mode”. I would like to elaborate a bit about the reasons for adding this and what it does.

The TL;DR is: use “Medium Distortion, OK Frequency Response”.

A bug report I received stated that QRange shows rather high distortion when used in linear phase mode while all was fine in minimum phase mode. This was especially the case when using high Q-factors and boosts as well as low frequencies. After some investigation I found the culprit.

As explained in IIR Linear Phase EQs post QRange uses normal EQ filters but processes a block of samples forwards and backwards, resulting in a linear phase response. As each block is processed independantly some overlap must exist between the blocks to allow fading. In QRange this is done by applying a hann window to the block. Originally, this was done after processing the filter. That is, a block of samples was taken, the filter was applied followed by the window. These blocks were then put together again. I also experimented with applying the window before processing the filter but QRange’s frequency response looked better if the window was applied after the processing. I did not look at distortion effects.

As it turns out this is not ideal distortion-wise. The reason is that the filters have to be reset for every block and will thus start cleanly. Filters always take past samples into account. While the incoming signal is continuous it will be chopped into blocks that – for the filter – start from silence. This more or less results in a step response. As one can see in the wikipedia article a step response can have some ringing depending on the filter’s Q-factor. The ringing’s frequency also depends on the filter’s frequency. If the Q-factor is high and the block size is short compared to the filter’s frequency the ringing will add significantly to the signal resulting in the seen distortion. The distortion is also not the nice kind of distortion one gets in transformers or tubes but a rather ugly distortion like granular distortion due to quantization as the added signal isn’t that much related to the input.

To counter this effect the “Process Mode” option was introduced. The available options are:

  • Low Distortion“: Windowing is done before the filters are processed.
  • Medium Distortion, OK Frequency Response“: Windowing is split before and after the filter processing.
  • Good Frequency Response“: Windowing is done after the filters are processed.

The last option is compatible with previous versions of QRange and will be used if an instance is loaded that was saved before the 1.0.4 update. As said, the other options’ frequency response isn’t as good as the third option’s one. This is comparable to using a shorter latency setting.

Normally “Medium Distortion, OK Frequency Response” should work best. If only small boosts/cuts are used at low frequencies (say +/- 6 dB max) the “Good Frequency Response” option is still a viable choice. “Low Distortion” can be used alongside a long latency setting for severe bass processing.

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