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USB DAC GUIDE TO COMPUTER-AUDIO | 12.2015 | 16
AUDIO TRANSMISSION MODES
Looking for bit perfect sound! – Audio transmission modes explain in details
Windows: direct sound
It’s the standard playback mode on a Windows PC.
It goes thru the kernel mixer, that acts as an interface of the OS kernel between
the various sound sources in a PC (PC sound, MIDI, players, etc…) and the sound
card’s driver. All the various sources can produce sounds with dierent sampling
frequencies; the kernel mixer will resample all the various audio streams to a
unique sampling frequency and allows them to be mixed before sending them to
the audio card. But this mix will be at a low frequency (standard 44.1 kHz). On top
of this the mixer will add some treatment (dynamic compression, tone control,
loudness control, etc…) so this will generate a low sound quality.
For this reason, we should avoid direct sound mode for the best sound quality.
This is not bit perfect
Windows: kernel streaming
Kernel streaming is the “audiophile” playback mode for Windows. Standard play-
ers such as WMP or iTunes do not support this mode at the moment, so a dedi-
cated media player is required. The audio data will be sent from the media player
directly to the USB-DAC, by-passing the audio mixer. This will also need less
CPU power. So your PC will be more stable and generate less Jitter (less errors).
Now we have a so-called bit perfect sound. Kernel Streaming is often used on
PCs running Windows XP.
Windows: WASAPI
WASAPI (Windows Audio Standard API) was developed for Windows Vista,
Windows 7 and up to replace Direct Sound. WASAPI isn’t intended to work on
Windows XP. Standard WASAPI will make use of the Windows Sound Mixer and
is not bit-perfect.
Windows: WASAPI Event Style
WASAPI Event Style is the bit-perfect version of WASAPI and will by-passed the
audio mixer. The sound quality obtained using WASAPI Event Style is comparable
to Kernel Streaming mode. WASAPI needs a bit more CPU power than plain ker-
nel streaming. But, WASAPI gives access to a bigger choice of players. WASAPI
Event Style is bit-perfect.
Windows: ASIO and ASIO4ALL
ASIO means Audio Stream Input / Output and is another method to bypass the
Audio Mixer and in addition the Kernel, to let the media player directly talk to
the USB-DAC. ASIO was built for XP but it works on all OS. ASIO was created to
overcome the USB Audio 1.1 limitations. On USB 1.1 only 48 kHz/16bit in direct
sound mode was possible, but with ASIO 96 kHz/24bit.
ASIO is bit-perfect.
Mac OS: Core Audio
Core Audio is the Mac equivalent for the Windows Audio Mixer and handles any
kind of audio information – input and output.
Core Audio is not bit-perfect
Mac OS: “hog mode”
The dierence between Core audio and hog mode operation is that Core audio
allows more than one client to access the driver at the same time. The Core
more mode will have the same issue that we already see on the Direct Sound
streaming in windows so we will lose sound quality. To avoid this, on Mac we
will use the “hog mode”. The “hog mode” needs to be activated in the Media
Player. The mode will allow the player to get first priority when started and block
all other audio devices, so in this case we will get the best sound without any
mixing processing.
Mac Integer/mode
Addressing directly the HAL [AppleHAL_2] gives the possibility to bypass the
two main overhead
Processes of the above standard mode:
• Mixing buer
• Float to DAC native format conversion
In Integer Mode the player software supplies a stream already formatted in the
native DAC format.