Synchronous Sample Rate Converter (SSRCs) is used to convert the input sample rate to the required output sample rate.
SSRCs are core objects that can operate within an audio core. Currently there is one implementation of SRCs in Awx.
Two options are provided to convert the sample rate. Both these options are mutually exclusive.
IIR Integer Multiple SSRC
This core object implements a synchronous sample rate converter whose input sample rate / input block length and output sample rate / output block length are integer multiple of each others. This is also an infinite impulse response implementation (IIR).
The object operates in one of 2 modes:
- User Coefficients mode
- Predefined Coefficients mode
Before we get into the details, there are some common configuration parameters between the two.
- The input block length needs to be set by the user.
- The Biquad filter topology. Currently 2 topologies are exposed.
- Direct Form I
- Direct Form II
User Coefficients mode: In this mode, the user has to provide the input and output sample rate. Input and output sample rates should not be equal. The Number of Biquads field is read-only.
User has to import the coefficients by clicking on the button “Import Co-efficients”. Based on the number of coefficients in the file, the Number of Biquads is updated.
Validations for User Coefficients mode: The Input and Output sample rates cannot be the same. Validation is shown when the same values are entered.
After adding a new “Ssrc lir Int” object and selecting “User Coefficients Mode”, if the coefficients are not imported, the following message will be displayed on various operations such as “Save”, “Edit Device”, “Copy Core Objects” and “Paste Core Objects”. After importing coefficients, the user can perform the required operation.
Predefined Coefficients mode: In this mode, the xAF dll is used to read the input sample rate, output sample rate, and the number of biquads. When a value in the combo box is selected, the xAF dll is also used to fetch the corresponding coefficients.
Biquad Co-efficient has to be re-imported whenever the mode is switched between Predefined Co- efficient mode to User Co-efficient mode.
For these pre-defined coefficients, the quality measures are as follows:
- Signal to noise ratio: 80 dB
- Total harmonic distortion: 2e-3f
- Spurious free dynamic Range: 59 dB
- Total harmonic distortion plus noise: -60 dB
- Frequency response flatness: 3 dB
Output block length (Displayed as a read-only field) = (Output sample rate /Input sample rate) * Input block length.