Sonifex avn PC-AD2, Handbook

6

3       PC-AD2 - Windows Audio Operation

3.  Windows Audio Operation

The audio subsystem of Windows 7, 8, 10 and corresponding server 
versions changed substantially from earlier versions of Windows, although 
most applications should continue to work without modification. The major 
differences likely to affect users of the PC-AD2 are described below.

Audio Endpoints

Key to the new system is the concept of 

audio endpoints

. These are the 

physical audio sources and destinations, such as microphones, speakers and 
line connectors.

Previously, an audio capture device would typically have multiple inputs 
which were combined in a mixer or selector. Now, each input is represented 
by its own device, and Windows itself operates the card’s selector to route 
the chosen source to the A/D converter.

In the case of the PC-AD2, each of the physical line inputs is represented 
by an endpoint device, and as the hardware has separate A/D converters 
or AES-3 receivers for each one, they can be used simultaneously. A 

Microphone 

endpoint is also created which is shared with analogue 

Line 

Input 1

, so only one of these can be active at any given time. When an 

application opens a shared input endpoint, it is automatically activated and 
the other shared endpoint is made unavailable.

Windows has a default playback endpoint and a default recording endpoint, 
which are typically used by applications where the input or output device 
cannot be explicitly chosen. To change the default endpoints, right-click on 
the loudspeaker symbol, selecting 

Playback devices or Recording devices

, 

then right-click on the desired endpoint and select 

Set as Default Device

. 

The default device is remembered across reboots and should not need to 
be set again. A default communications device can also be set for use by 
telephony applications.

Mixer API

The legacy Windows mixer API is virtualised for each application, providing 
just a mute and volume control for each endpoint and affecting only the 
audio going to and from that application. The hardware controls on the card 
itself are not accessible from the mixer API.

New applications can use the Core Audio APIs to directly access 
the hardware mixer controls and peak meter nodes through the 
DeviceTopology API and the EndpointVolume API (which includes the 
IAudioEndpointVolume and IAudioMeterInformation interfaces). These are 
documented in the Windows SDK available from Microsoft, however note 
that these APIs don’t work with versions of Windows earlier than Vista.

Audio Engine

The Windows audio engine runs at a fixed sampling rate and bit depth. 
The PC-AD2 driver sets the sampling rate to the card’s core rate and the 
bit depth to 24-bit for all the inputs and outputs. The Windows sampling 
rate converter is used whenever a different sampling rate or bit depth is 
requested by applications. The engine bit depth for each endpoint can 
be changed to 16-bit if required by selecting the endpoint, clicking on 

Properties

, and then clicking on the 

Advanced 

tab.

If your applications will be playing or recording at rates higher than 48kHz, 
you will need to set the card’s core rate accordingly otherwise Windows will 
down-sample to 48kHz.