This document applies to the Windows drivers for Prism Sound USB interfaces (currently Lyra, Titan and Atlas) starting with driver version 1.05, released on 17th July 2014.

The performance and capabilities of the new driver have been significantly increased, as described below. These changes result in a lower latency (delay) through the driver, and even more reliable operation with a wide range of PCs.

The treatment below applies to Windows PC's only and is not applicable to MAC users as the MAC OS handles latency optimization internally. For a more general treatment of the issues on Windows, please also see the guide to "Optimizing your Windows PC for audio".

Background

The trade-off between latency (delay) and reliability (click-free operation) for computer audio interfaces is well documented (see, for example, the Lyra online manual latency section.).

Manufacturers of computer audio interfaces (and software) have traditionally dealt with this by having a user-settable control to adjust the latency, with an implied improvement in reliability as the latency is increased.

However, this approach has begun to appear simplistic, since there are a number of different factors which can affect the threshold of reliable operation on Windows PCs, and it is increasingly unsatisfactory to try to address all of these factors optimally with a single control. This is especially true for 'professional' soundcards, where some users want minimal latency and are prepared and able to fine tune their PC to achieve it, whereas others don't care about latency, and simply want to plug the interface in and never hear a dropout.

The new driver

So, in trying to address a wider range of user requirements, we have changed both the internal workings of the driver, and also the available controls.

The resulting Windows USB audio driver version 1.05 can achieve reliable operation with very low latencies on fast, well set-up PCs, but can also work reliably on very slow and busy PCs.

There are now two controls that set the buffer size:



'ASIO' is the buffer size that the ASIO driver will use, expressed as a number of samples.

'Base' is the 'Base resolution' of the USB audio packets: the duration of a single packet, expressed in ms.

Generally speaking, smaller ASIO and Base values result in lower latencies. Round-trip latency (RTL) times for particular settings are reported in your ASIO DAW. Conversely, larger ASIO and Base values result in increased stability and resilience to high CPU load and extraneous activity.

Base resolution - a new setting

The Base resolution can be selected as 1, 2, 3 or 4ms. You should regard this setting as a starting point depending on the performance of your PC. 4ms for an abnormally slow PC or netbook, 1ms for a fast, tweaked PC used for pro-audio applications where low latency is important. 2ms is best for most modern PCs in most situations.

When you change this setting, your interface will briefly disappear from the USB whilst its buffering parameters are revised, and will then reappear with the new settings. You may find that the ASIO buffer size has changed to a new minimum value.

The base resolution will be displayed in red when the 1ms setting is selected to indicate that the system will be susceptible to DPC spikes. DPC spikes are periods when the CPU is unavoidably occupied handling Deferred Procedure Calls. If the DPCs from other processes (particularly poorly written drivers) take too long and the audio buffer is depleted there will be drop-outs in the audio.