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Technical Guide for Playback and Recording

Technical Guide for Playback and Recording
Tweaking kX Driver for ultimate performance

Status of this document: DRAFT
Comments and bug reports are welcome

Introduction

kX-compatible audio cards are based on the E-mu 10k1 and 10k2 processors, which are quite complicate devices. Using all the features of these audio cards requires certain level of understanding of their internals.

Installation and system requirements

First, you need to check if the kX Audio Driver has been installed correctly and is functioning properly. Right after reboot, you should see kX Tray icon in your System Tray. Also, check 'Control Panel' - 'Sounds and Audio Devices' - 'Audio' tab: for each Wave device click 'Advanced...' button and check if 'Speaker set-up' is set to '5.1 surround sound speakers', 'Hardware acceleration' is set to 'Full' and 'Sample rate conversion quality' is set to 'Best'.

kX Mixer automatically configures all these parameters, except 'Hardware acceleration', since, due to Microsoft limitations, this option cannot be changed by Software. Most OSes set 'Hardware acceleration' to 'Full', however, Windows 2000 and some other OSes use different default values.

Why should you set 'Sampling rate conversion quality'? kX Audio Driver does NOT provide any software Sample Rate Conversion and only uses hardware-accelerated features of your audio hardware. That's why any unsupported feature will be implemented in Software, in general, by the KMixer (Kernel Streaming Mixer) component of your OS (a part of DirectSound). This option ensures the best algorithms are used for audio conversion.

kX Audio Driver exposes different audio devices: Wave 0/1, Wave 4/5 etc... Any reference to 'Wave 0/1' usually applies to any 'Wave x/x' device, but not necessarily to 'Wave HQ'. For additional information on 'Wave x/x' assignment, please review our 'Routings Guide'.

The best results can only be achieved under Windows 2000, Windows XP and later OSes. Windows 98SE and Windows Millenium have certain bugs in the KMixer and DirectSound implementations, causing severe audio distortion (in particular, -6dB software-based attenuation).

Playing back 24/96 content

The only card that supports 'native' 24/96 playback is Audigy2. kX Audio Driver (version 3537 and later) fully supports this feature. However, you need to keep in mind that 24/96 functionality is implemented in a 'tricky' way due to hardware design. Since the DSP is still operating at 16(24)/48, all the 24/96 content is played back via a separate hardware chip called 'p16v' (Some additinal information can be found in our Audigy2 / P16V Routings guide). The audio data gets directly to the i2s / spdif codecs and interfaces and is NOT processed by the DSP. However, the p16v output is also routed to the 10k2 chip and is available in the DSP by means of the 'p16v' plugin. Note that the incoming 24/96 audio data is resampled before it gets into the DSP. Both 10k2 and p16v output streams are mixed (and re-sampled when necessary) before they get to the i2s / spdif codecs and interfaces. (More information on i2s / spdif assignments can be found in our kX IO Assignments guide).

So, for instance, when you playback any 16/48 content via 'generic' audio devices (such as "Wave 0/1"), the audio stream is processed by the DSP and goes to the p16v mixer. If there is no simultaneous 24/96 playback, the audio data is passed to the i2s / spdif codecs and interfaces. NOTE: spdif frequency depends on user setting (found on the main page of the kX Mixer), so, if you set SPDIF frequency to, say, 96000, the data is resampled. Also note, that i2s codecs always operate in 24/96 mode by default (and the preset driver implementation doesn't change this setting).

For 24/96 playback, kX Audio Driver exposes a special 'Wave HQ' device. When it is active, the audio stream goes to the p16v device and, optionally, to the DSP (resampled to 48000 frequency, the bitness of the incoming signal is unknown, but is probably 16 or 24 bit). If you start simultaneous 16/48 playback or use the 'p16v' plugin in the DSP, the 10k2 output is automatically resampled to 24/96 and is mixed with the original 24/96 content.

That is, you can, for example, use the 'Wave HQ' device for high-quality 24/96 playback, while using the DSP for, say, Reverberation processing (in 16/48). Both audio streams will be mixed and will be sent to the i2s codecs (in 24/96) and to the spdif interface (resampled to user-selectable frequency). Please note, that the DSP output is always 24/48 (not 16/48) for any outputs except AC97.

For measuring 24/96 performance in 'Digital' mode, make sure to set the appropriate SPDIF Output frequency in the kX Mixer. For 10k2-based boards, the SPDIF interfaces are always configured to produce 24-bit output.

Please keep in mind that while measuring 'Wave HQ' performance (24/96 playback), the DSP is still functioning and might affect the results. So, you may need to disable the Reverb effect and ensure the output is '-inf' (by opening the 'Peak' plugin).

Disabling Reverberation: Since reverberation algorithms usually produce infinite response ('reverb trail'), simply setting Reverb level to '0' won't give you '-inf' at the outputs. You will need to manually disable the particular Reverb effect in the kX DSP window.

Audio Interfaces

There are at least five different audio interfaces supported by the kX Audio Driver:

  • WinMM
  • DirectSound
  • Kernel Streaming
  • ASIO
  • GSIF

Each interface has its own limitations and features. Some of them come from the nature of the particular interface, while the others are hardware-related.

Kernel Streaming interface is used by Sonar, Cakewalk, Foobar2000 and other applications. This interface passes the data directly to the driver and bypasses any OS audio processing layers. This ensures the audio card is operating in the desired mode, however, if the particular audio format is not supported, an error will be generated. 'Native' hardware-accelerated formats for 'Wave 0/1' device are: 16 bit, 100Hz..191199Hz. Any incoming audio data is automatically resampled (in hardware) to 16/48 format. Since software resampling algorithms might give you better audio quality, it is recommended to set your audio players to 16/48 format and use software SRCs.

For 'native' 24/96 playback (Audigy2 only), the only format supported in hardware is '24 bit padded to 32 bit' (sometimes called 'Alt 24 bit'). If possible, you should set this format in your audio application. That is, the hardware (and, thus, the driver) don't support '24-bit' playback, if the audio sample occupies exactly three bytes. Since some audio applications fail to initialize 24-32 audio format properly, the very recent kX Audio Driver versions support a kind of '32-bit' audio format. Of course, since the hardware doesn't support 'true' 32-bit playback, the driver uses only the high 24 bit of the incoming audio data.

The 'p16v' device supports 24/96, 24/48, 24/44.1 and 24/192 formats only. Any other sampling rate will be rejected, if using Kernel Streaming interface. NOTE: support for 44.1 is experimental and might produce incorrect results for certain card models.

WinMM interface is used for Windows sounds and can be used as the default output device for most audio applications. This interface is shared across all the applications, that's why its functionality is rather complex. By default, kX Audio Driver initializes WinMM interface to operate at 16/48 for 'Wave 0/1' and at 24/96 for 'Wave HQ' devices. The audio application can change the output frequency, but only if it is the only running audio application. Otherwise, software mixing and optional resampling is performed by the KMixer component of the Windows Kernel.

24/96 hardware implementation supports only one hardware stream at the moment (note that hardware stream can be multichannel: that is, you can have one stereo or one 7.1 audio stream, but not, say, two stereo streams).

WinMM interface will provide automatic format conversion for hardware-unaccelerated audio streams. Keep in mind that this conversion is performed by the Microsoft KMixer component, not the kX Audio Driver. Under certain conditions this can cause problems: for instance, playing 24-bit audio on a 'Wave 0/1' device might give you additional noise introduced by the KMixer. (For instance, try to play digital silence and watch the peak meter -- don't forget to disable Reverberation).

DirectSound interface not only functions independently, but also serves as a layer between WinMM and Kernel Streaming interfaces.

According to the Microsoft specification, the WinMM/DirectSound subsystem always allocates the best possible audio stream and sets the maximum supported format. After this voice is allocated, it is used both for WinMM playback and Direct Sound software buffer. Since the best audio format supported by the card for 'Wave 0/1' device is 16/19199, in order to avoid WinMM playing back at 16/19199 by default (with intensive software and hardware resampling!), kX Audio Driver exposes a different format, 16/48, that is used as the default one.

Most DirectSound applications allocate additional voices (streams), that are used for actual audio playback, while the voice, initially allocated by the KMixer / WinMM subsystem, is kept inactive. These voices are always configured to the required sampling rate, thus giving optimal performance and hardware-accelerated SRC.

That is, when playing back via WinMM, the actual sampling rate depends on the application settings and any possible WinMM/DirectSound applications running in the background. If the application is the only one accessing the device, dynamic hardware-accelerated sampling rate change is usually possible*. If there are additional DirectSound/WinMM clients running, the final WinMM sampling rate will be probably 16/48 (with all the rest WinMM / DirectSound (Software!) streams re-sampled to 16/48).

* The very recent driver releases force any WinMM stream to play at 16/48 for 'Wave x/x' devices. Of course, dynamic sampling rate change is still supported for DirectSound clients (unless they specify 'Software' buffers). That is, if you are using 'WinMM' interface for your audio tests, keep in mind that switching sampling rate will measure KMixer algorithms and not the hardware SRC quality!

This way of operation functions correctly for 'Wave 0/1' device, but causes certain issues for the 'Wave HQ' device.

Since the 'p16v' hardware doesn't support multiple independent voices, the driver informs the KMixer component of the unique voice available. In this case the situation is reversed: since DirectSound cannot allocate additional audio voices, it uses the 'initially allocated' voice for all the audio playback, and the audio data is passed through KMixer (and is resampled and format-converted).

The kX Audio Driver enables WinMM interface to change the sample rate for 'Wave HQ' device (44.1, 48, 96 or 192 kHz). That is, any WinMM audio stream (if it is the only one currently active) will utilize hardware-accelerated SRC, while any DirectSound client will cause the driver to switch to 24/96 mode.

There's no easy alternative for the present mechanism at the moment. Please keep this functionality in mind when configuring your audio software.

In any case, the information about the actual format and sampling rate of the audio stream is displayed by the debugging tools.

ASIO and GSIF interfaces currently operate at 16/48 (also at 16/44.1 for ASIO, playback only). There will be a separate ASIO driver for 24/96 playback and recording in the future (of course, for Audigy2-only based cards).

24/48 and bit-to-bit Playback

kX Audio Driver supports a special 'trick' in order to support 24/48 playback via the DSP for Audigy and Audigy2 cards. In order to use this feature you will need to set your audio player to 24/48 format (any other sampling rate is not supported!) and replace the 'FXBus' plugin by the 'FXBusX' plugin in the kX DSP. In order to check, if the audio playback is really performed in 24/48 format, open the 'Analyzer' window of the kX Mixer: you should see 'Dark Green' strips (while 'generic' audio streams are colored red). In most cases, this 'trick' doesn't affect any audio applications configured to operate at 16 bit. However, the 'FXBusX' plugin affects incoming audio signal. Please review 'DSP Resampling' below.

Please note, that the only device that supports this 'trick' is 'Wave 0/1'. Also, your application should be using DirectSound interface, since WinMM streams will be automatically converted to 16/48 by the KMixer.

DSP Resampling

It is widely known that 10k1 and 10k2-based audio cards perform audio resampling even when the incoming audio signal is 16/48. This happens due to not-so-perfect implementation of the SRC algorithms in hardware. For Audigy and Audigy2 cards (and, probably, for 10k1-based cards with chip revision >= 7 as well) the 'modified' 16/48 audio stream can be restored by using 'b2b' or 'FXBusX' plugins in the kX DSP.

The nature of the SRC bug causes all the audio data to contain partially-wrong 16th bit, thus giving you '15.5-bit playback' (and not '16bit'). The known solutions (the DSP plugins mentioned above) restore the 16th bit of the audio data, but only for the original 16/48 content. That is, if the incoming signal is not 16/48, but is, say, performed at 44100, 22050 or any other frequency, the 'b2b' and 'FXBusX' plugins, obviously, won't "correct" it, but will change the signal a little bit. That's why this correction is not turned on by the kX Audio Driver by default.

What is the difference between using b2b and FXBusX?(advanced topic)
The main difference is that FXBusX not only restores the 16th bit, but also performs sound truncation to 24 bits (optionally obtaining the least 8 bits from FXBus2 sources, if available), while the 'b2b' plugin restores the 16th bit only. From the user point of view this causes the following problem: if the truncated audio data generated by the FXBusX plugin is passed thru the Routing and Epilog plugins, it is automatically restored to '15.5bit' state (due to mathematical conversion), while the 24-bit audio data is not. So, when using FXBusX for bit-to-bit 16/48 playback, it is preferred to avoid adding any volume controls and route the output directly to the epiloglt (lite version!). The 'B2B' doesn't truncate the audio data and can be easily inserted between, say, FXBus 0,1 and the Routing.

The output signal (for instance, of the SPDIF outputs) might get truncated / rounded by hardware. This option is card-dependent (certain cards perform that, while the others don't). That's why it is recommended to check bit-to-bit playback and the particular B2B/FXBusX chain before using it (for instance, by trying the 'Direct SPDIF Recording' method and a SPDIF loopback cable).

Volume Levels and kX DSP settings

In order to avoid any software processing and audio quality loss, the volume levels should be set as follows:

  • Master Level: 100%
  • Wave level: 100% (both, in kX Mixer and in your audio application)
  • Master Recording level: 100%
  • All inputs and outputs are to be set to '0dB' or muted
  • To avoid recording 'What U Hear', set Wave, Synth, FX1 and FX2 Recording Levels to -inf

In Direct SPDIF Recording no kX Mixer level affects the signal. In 'p16v' 24/96 recording, only the 'Master Recording Level' affects the signal.

When testing audio quality, it might be necessary to unload 'Surrounder' effect, set it into '5.1 Direct Path' mode, or set it into 'bypass' mode.

NOTE: if you still need to change the volume (e.g. in order to set RMAA levels to -1.0), in any playback mode please use 'Master Volume' since it is the only level that is always hardware-accelerated.

Recording

Direct SPDIF Recording

some information is already available in our Direct recording guide. certain additional stuff to be added

Bit-to-bit Wave recording

[in short]: FXBus - b2b - recl
TO BE WRITTEN

24/96 Recording

TO BE WRITTEN

It seems that in order to achive best performance when recording in 24/96 (analog inputs only) you need to lower the incoming signal a little bit(NOT the 'Recording level'), otherwise clipping and distortion can occur.

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