Basic Signal Flow
The Oxford SuprEsser contains a pair of mutually opposing filters – by default one is a narrow band pass filter, the other is the complementary narrow band reject filter. This gives one signal path containing just the contents of the band of interest, and another signal path containing the input signal with this band entirely removed. When mixed back together in equal ratios, you get the original signal.
The band pass signal is usually fed to the compressor, both to the side chain and to the main signal, so that it is just this signal that triggers gain reduction, and it is just this signal that is affected by any gain reduction.
The band pass/reject filters will switch automatically to other EQ types when the conditions warrant them: giving a total of 4 EQ types. When the “Width” control narrows the band pass filter width to its minimum, a High-Q notch filter is invoked that gives much better reduction of a very narrow band of frequencies. When either side of the band window touches the end stops (20Hz or 20KHz), the band pass filters change to LFCut or HFCut as necessary.
Advanced Modes Signal Flow
In addition to 4 EQ types, there are 4 different compressor modes in relation to what signal is passed to the side chain and what signal is passed to the main input of the compressor. See the section “Operational Modes” for more information on this.
The following diagram gives a more complete signal flow with regards to the advanced modes of operation.
The Wet/Dry control is a much requested feature that allows you to add back in the uncompressed signal to a highly compressed signal to add back in some punch. As you can see from the signal flow above, the blend control is implemented inside the dynamics module, and therefore only operates as expected when you are listening to the “Mix”, or “Inside”.
Resolution, Kernel Sizes, Delays and Buffer Sizes.
The linear phase filters used by this plugin require an “Impulse Response Kernel” to model the response of the internal Oxford Filters. The size of this kernel (as measured in samples) determines both the plugin delay, and the accuracy of the modelling, which affects the performance especially at lower frequencies.
Larger kernels mean: a) Better performance at low frequencies, but b) Longer overall plugin delays.
Smaller kernels mean: a) Adequate performance at high frequencies, and b) Smaller plugin delays. While small kernels are adequate for high frequency work such as de-essing, the performance at low frequencies will cause poorly defined filter slopes, and poor separation of “Inside” from “Outside”.
We have found that hosts are generally not able to readily adjust their delay compensation engines if the kernel size is adjusted dynamically at run time. Therefore, the Oxford SuprEsser is released as three separate plugins, each with a different fixed kernel size.
The standard version has a kernel size of 2048 samples and is called the “Oxford SuprEsser”. This is suitable for work across the entire frequency range at 44.1kFs, and thus is the standard plugin to reach for, especially for the ”Mixing” stage of a project. Depending on your audio buffer size, the delay or latency will be somewhere in the region of 1044 to 3092 samples. See the section below for information on reducing the delay.
The large kernel version has a kernel size of 8192 samples and is called the “Oxford SuprEsser HR” or “Oxford SuprEsser HighRes”, depending on your host. This version gives superior resolution at the lowest frequencies, and thus is particularly suitable for LF mastering work. Remembering that as you increase the sample rate, the resolution at the LF end will be correspondingly reduced, this version is particularly suitable for use at higher sample rates, especially 176.4k or 192k. This version will have a very significant delay that can be beyond the ability of Protools HD to automatically compensate for. The true delay is reported correctly in the track delay information and depending on your audio buffer size, the delay will be somewhere in the region of 4116 to 12308 samples. See the section below for information on reducing the delay.
The small kernel version has a kernel size of 512 samples, and is called the “Oxford SuprEsser LL” or “Oxford SuprEsser LowLatency”, depending on your host.
This version gives superior performance in terms of having a small delay, and thus is more suitable for live DeEssing work at low sample rates, and for the “Tracking” phase of a project where you are laying down new tracks from midi instruments, and you don't want a significant delay between what you are playing and what you are hearing. The smaller kernel size means the resolution at low frequencies will be poor, and so cannot be used much below 400Hz. Depending on your audio buffer size, the delay or latency will be somewhere in the region of 276 to 3072 samples. See the section below for information on reducing the delay.
Delay Compensation and Audio Buffer Sizes
The plugin delay produced by the Oxford SuprEsser depends both on the Kernel size/Resolution and the audio block size.
The block size is the size of the sample buffers passed to the plugin by the host, and is usually specified in your audio hardware preferences/configuration/setup page. The reason that the plugin delay depends on the block size is that the plugin must accumulate a whole kernel sized block of samples before it can process them.
In order to ensure the minimum plugin delay, make sure the block/buffer size is the same as or greater than the Kernel size/Resolution setting!
For example, if the kernel size is set to 512, i.e. you are using the Low Latency version, then if your block/buffer size is 512 or 1024, this will ensure the plugin produces the minimum delay of 276 samples.
If you use a smaller block size than the kernel size, the overall delay of the plugin will go UP, not down. For example, if the block size is 256, then the Low Latency version will produce a delay of 532 samples.