This article explains some of the reasons for crackles, dropouts (gaps in playback) or glitches during playback. Hereafter these will be referred to as simply ‘dropouts’. It also tries to explain why a modern computer can have performance problems although it is powerful enough for the task. It explains the problem domain and tries to give some tips on how you can solve the problems.
In some cases, your USB bandwidth might be limited, and this could introduce various problems. There are a couple of reasons why those problems may arise.
Sharing USB Host controller (USB 2.0 Full Speed)
Overbridge for Analog Four MKI, Analog Keys, and Analog Rytm MKI uses USB 2.0 Full Speed. This USB specification limits the data transfer rate to max 12 Mbps. Later Elektron products use USB 2.0 High Speed and have a max rate of 480 Mbps. Many laptops use a single USB host controller that controls all ports. It means that if you have multiple devices connected that run with USB 2.0 Full Speed they might be sharing the 12 Mbps maximum bandwidth. It is, however, possible that your computer has multiple USB host controllers. You can try to connect a device to a different port that might also use a different host controller internally. This might increase the possible bandwidth for each device.
USB 2.0 Full Speed also limits the number of audio channels that can stream simultaneously. If you open the Overbridge Control Panel and configure your USB Full Speed device to use fewer channels and/or a lower bit depth, this might improve the Overbridge performance on your system.
When connecting a USB 2.0 Full Speed device (12 Mbps) to an STT hub (typical for an internal laptop hub), all devices on that hub are limited by 12 Mbps, even devices that are otherwise High Speed or Super Speed. We recommend that you use an MTT hub (such as the Elektron Overhub) whenever you connect a Full Speed device to your system so as not to constraint your other USB devices.
It is especially important to use an MTT hub if you want to use more than one Full Speed device at the time. Connecting two Analog Four MK1 to an STT hub, for example, causes them both to share only 12 Mbps bandwidth. If they instead connect via an MTT hub, they get 12 Mbps each.
SAMPLE BUFFER SIZE & AUDIO LATENCY
The audible latency in your DAW can be described as the delay time from when the source of an audio stream is played until you hear it. The sample buffer size is the number of audio samples that are processed in each audio packet. A larger buffer size introduces a higher audible latency but might solve problems related to real-time performance as the computer gets a longer time slot to process data. A system with less processing power or more other software/hardware needs a larger buffer size. If you experience dropouts you might have to adjust this to your needs.
Make sure your DAW is configured with an appropriate sample buffer size for your hardware.
Please also make sure that Overbridge is configured with an appropriate Plugin Buffer Size in the Overbridge Control Panel.
If you are on a Mac, adjusting the Buffer Safety Margin in the Overbridge Control Panel can also improve the performance of streaming audio.
If you are using Windows, adjust the performance mode in the Control Panel to match your hardware.
OTHER CPU INTENSIVE APPLICATIONS RUNNING
Do not run other CPU intensive applications at the same time as Overbridge. These might put a high strain on your computer and limit its real-time capabilities.
Examples of these kinds of applications are:
Web browsers with multiple open tabs.
Image or video/3D editor programs.
AUDIO LATENCY AND REAL-TIME REQUIREMENTS
Real-time applications, such as a DAW, have very different requirements than other software on your computer. The big difference between a DAW and for example, a video editing program is that in a video editing program the editing is mainly done while the video isn’t playing. After the editing is done the result is played. The processing is allowed to take some time and can be interrupted without you knowing that it was. This is not a real-time process.
Another example is a web browser that is allowed to have a response time from click to result of a few hundred or even thousands of milliseconds before it is deemed unusable. The typical case in a DAW, however, is to run a set of plugins and/or recording sessions while also listening to the output of these. The processing of these sources is always time-limited because if something takes a too long time, it is audible, and the harmonic content becomes distorted and unacceptable. This is a real-time process.
The sound card converts digital samples to analog from the sample buffer at a fixed frequency, decided by the sample rate in your DAW. It always expects there to be a sample in the buffer to convert. If there is no sample available, it tries to convert empty data, resulting in a dropout.
It is the CPU of your computer that carries out the actual audio sample processing. The CPU is, at the same time, constantly interrupted by other processes or devices in your system. Therefore, it can’t guarantee to deliver at a specific frequency but is instead given a time slot to deliver. This time is decided by the buffer size and sample rate in your DAW. Audio latency is when the CPU fails to deliver a sample to the buffer in time before the sound card needs to read it.
The CPU itself is capable of delivering in time on all modern high performing computers. However, as mentioned before, the CPU is always interrupted by other processes or devices on the system. This is not a problem per se, but some devices are more prone to lock up the CPU and not giving it enough time to perform the real-time task of audio processing. Other performance heavy software might also cause the same problem. Therefore, make sure you are not running any heavy processing outside of your DAW.
Listed below are some devices that can easily lock up the CPU.
High probability of lock-up
Medium probability of lock-up
Before troubleshooting the devices, make sure you have all of these devices’ latest drivers.
On a Windows PC you can debug your system by disabling devices in the device manager to figure out which device is causing the CPU to lock up. You can use tools such as DPC Latency Checker or LatencyMon to measure the devices’ latency independently from your DAW setup. If you do find problems when you run these tools, we recommend replacing the device or devices that are causing the problems. If you find no problems with either of these tools, there is a possibility that the problem is with your DAW or a plugin used in your project.
COMPUTER HARDWARE RELATED ISSUES
There are som known incompatibility problems with certain hardware:
Intel Core2Duo processors, or Xeon CPUs from that same generation, are not supported.
There could be USB 2.0/3.0 compatibility issues. For more information, please see this article.
Some USB host controller chipsets have worse performance than others. If you use a USB hub, it adds another chipset in the signal chain and could potentially worsen the performance.