General information about audio and computers About latency On any personal computer system, there is a delay between the moment you input a sound, or “tell” the hardware to play a sound, and when you actually hear it. This delay is referred to as the “latency” of the design. This imposes a problem for any system where you want real-time user input to affect the sound. Why is there latency? All audio applications receive and generate their audio in chunks. These chunks are then passed on to the audio card where they are temporarily stored before being converted into regular audio signals. The storage place for these chunks are called “buffers” (an analogy would be a bucket brigade, where a number of people each have a bucket, and water is poured from one bucket to another to reach its final destination). The smaller the buffers and the fewer they are, the more responsive the system will be (lower latency). The general rules regarding the buffer size are these: • A small buffer size reduces the latency (the time it takes for the audio to “travel” from the audio interface input(s) to the application and from the application to the audio interface output(s)). However, a small buffer size also increases the DSP Load. Too small a buffer size setting could also make the sound crackle and distort. • A large buffer size reduces the DSP Load (allowing for more tracks to be played back simultaneously) and also ensures good audio quality. However, a large buffer size also increases the latency. A high sample rate will also reduce the latency. However, this will also raise the demands on the computer and its software. If the system can’t cope with moving the data to and from the buffers fast enough, there will be problems that manifest themselves as glitches in audio playback. To make things worse, audio playback is always competing with other activities on your computer. For example, a buffer size that works perfectly under normal circumstances might be too small when you try to open files during playback, switch over to another program while Reason is playing or simply play back a very demanding song. What is acceptable? On a regular PC, the latency can vary quite a lot. This is an effect of the fact that computers and their operating systems were created for many purposes, not just for recording and playing back audio. For multimedia and games, a latency of a 100 ms might be perfectly acceptable, but for recording and playing back audio it is definitely not! • PC audio cards running under Windows with a MME driver might at best give you a latency of around 160ms. • The same card with a DirectX driver running under Windows provides at best around 40ms. • A card specifically designed for low latency, with an ASIO driver under Windows, or a Core Audio driver under Mac OS X, can usually give you figures as low as 2-3 ms. This is definitely good enough for audio applications. That’s also why ASIO or Core Audio drivers are required to run Reason. ReWire and Latency When you run Reason as a ReWire Device, it is the other program, the ReWire host, that is responsible for actually rendering the audio and playing it back via the audio card. It means that any latency is present in the ReWire host. When Reason runs as a ReWire Device, what audio hardware you have, what audio driver you use, and the audio settings you have made in the Preferences dialog are of no importance at all! All audio hardware settings are then instead made in the ReWire host application. Note, however, that you may have to adjust the “External Sync Offset” in Preferences if Reason is synced via MIDI from the host application (see “External synchronization considerations”). For information on ReWire, see “ReWire”. Reducing latency There are a few general methods for making sure latency is as low as possible: Make sure you are using the latest version of the drivers for your hardware audio interface. Adjust the Sample Rate and Buffer Size parameters as described in “Audio settings”. Remove unnecessary background tasks on your computer. This might be any background utility you have installed as well as networking, background Internet activities etc. Optimize your songs. You might run into situations where you have to raise the Output Latency setting to be able to play back a very demanding song on your computer. Another option would be to actually optimize the song. See “Optimizing Performance” for details. Get a better audio interface. This is only required if you find that you need to increase Output Latency because your audio card can’t really cope with the songs you try to play. Get a faster computer. This is only required if you find that you need to increase Output Latency because your computer can’t really cope with the songs you try to play. About processors When you run Reason, the numbers of physical CPU cores and the CPU clock speed are major factors determining how many audio tracks and devices you can use at the same time. If you plan to buy a computer specifically for Reason, you could play it safe and choose a computer with at least a dual-core Intel i7 processor, or equivalent AMD processor, running at 2.0 GHz or faster. MultiCore processors will give better performance and are highly recommended. About RAM Another important performance factor is the amount of installed RAM in the computer. Generally, one could say: the more the better, especially if you’re running several applications simultaneously. To run Reason, a minimum of 4 GB RAM is required, but more is highly recommended for better performance.
