Update README.md
This commit is contained in:
Robin Davies
@@ -7,31 +7,32 @@ Use your Raspberry Pi as a guitar effects pedal. PiPedal allows you to control a
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PiPedal running on a Raspberry Pi 4 provides stable super-low-latency audio via external USB audio devices, or internal Raspberry Pi audio hats.
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PiPedal's user interface has been specifically designed to work well on small form-factor touch devices like phones or tablets. Clip a phone or tablet on your microphone stand on stage, and you're ready to play! Or connect via desktop browser. The PiPedal user-interface adapts to the screen size and orientation of your device, providing easy control of your guitar effects across a broad variety devices and screen sizes.
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PiPedal's user interface has been specifically designed to work well on small form-factor touch devices like phones or tablets. Clip a phone or tablet on your microphone stand on stage, and you're ready to play! Or connect via a desktop browser, for a slightly more luxurious experience. The PiPedal user-interface adapts to the screen size and orientation of your device, providing easy control of your guitar effects across a broad variety devices and screen sizes.
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PiPedal provides a simple configuration tool that allows you to easily set up a WiFi hotspot on your Raspberry Pi which can then be used to connect wirelessly.
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PiPedal provides a pre-installed selection of plugins from the ToobAmp collection of plugins. But it works with most LV2 Audio plugins. There are literally hundreds of free high-quality LV2 audio plugins that will work with PiPedal.
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PiPedal provides a pre-installed selection of plugins from the ToobAmp and Guitarix collection of plugins. But it works with most LV2 Audio plugins. There are literally hundreds of free high-quality LV2 audio plugins that will work with PiPedal.
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If your USB audio adapter has midi connectors, you can use midi devices (keyboards or midi floor boards) to control PiPedal while performing. A simple interface allows you to select how you want bind PiPedal controls to midi messages.
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## System Requirements
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* A Raspberry PI 4B or 400, with at least 2GB of RAM to run (4GB recommended).
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* To build PiPedal, at least 4GB (8GB recommended).
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* An external USB Audio Adapter, or a Pi audio hat with at least one audio input, and one audio output.
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PiPedal has been tested on the following Operating Systems:
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* Ubuntu 21.04 or later, 64-bit (recommended)
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* Ubunto 21.04 or later, 32-bit.
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* Raspbian 64-bit or 32-bit
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64-bit operating systems are preferred, as they delivery significantly higher performance than 32-bit platforms.
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If you are using Rasbian, make sure to upgrade to the latest version, because versions of the Linux kernel later than 5.10 provide important bug fixes for USB audio devices.
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## Choosing a USB Audio Adapter
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@@ -43,30 +44,6 @@ Pay close attention to the input VU meter of the first effect in your guitar eff
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Again, the MOTU M2 excels in this regard. It provides large, volume knobs for input and output, along with very readable VU meters which indicate both input and output signal levels.
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## Latency
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Note that Pipedal is not intended for use when logged in to Raspbian. Screen updates and heavy filesystem activity will cause audio dropouts. For best results, access PiPedal using the web interface remotely, through the Wi-Fi hotspot. Accessing the web interface via Wi-Fi has little or no effect on audio latency or dropouts.
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With a good USB audio device, PiPedal should be able to provide stable audio with 4ms (good), or 2ms (excellent) latency on a Raspberry Pi 4 when running on a stock PREEMPT kernel. Your actual results may vary.
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The current Linux kernel provides best latency on USB audio devices when they are configured with a sampling rate of 48kHz, and 3 buffers. Cheap USB audio devices (e.g. M-Audio M-Track Solo, available for less than $60) should be able to run without dropouts at 48kHz with 3x64 sample buffers. Most devices in this class use the same Burr-Brown chipset. Premium USB Audio devices should run stably at 48kHZ 3x32 sample buffers (about 2ms latency). I personally use the MOTU M2 USB audio adapter, which I highly recommmend -- stable, quiet, low-latency, great controls, and built like a tank).
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Make sure your system is fully updated, and that you are running with a kernel version of 5.10 or later, since version 5.10 of the Linux kernel provides improved support for class-compliant USB audio. The MOTU M2 (and many other USB audio adapters) will not work on versions of the kernel prior to 5.10.
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Prefer 64-bit operating systems to 32-bit operating systems. ARM processors execute 64-bit code about 40% faster than 32-bit code providing the same functionality.
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RT_PREEMPT realtime kernels (when available) are preferred but not required. (As if Februrary 2022, there aren't any good sources for latest versions of Ubuntu or Rasbian). PiPedal provides better (but not dramatically better) latency when running on a Raspbian Realtime kernel. Stock Raspbian provides PREEMPT real-time scheduling, but does not currently have all of the realtime patches, so interrupt latency is slightly more variable on stock Rasbian than on custom builds of Raspbian with RT_PREEMPT patches applied.
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The Ubuntu Studio installer will install a realtime kernel if there is one avialable. But -- at least for Ubuntu 21.04 -- there is no stock RT_PREEMPT kernel for ARM aarch64.
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On a Raspberry Pi 4 device, Wi-Fi, USB 2.0, USB 3.0 and SDCARD access are performed over separate buses (which is not true for previous versions of Raspberry Pi). It's therefore a good idea to ensure that your USB audio device is either the only device connected to the USB 2.0 ports, or the only device connected to the UBS 3.0 ports. There's no significant advantage to using USB 3.0 over USB 2.0 for USB audio. Network traffic does not seem to adversely affect USB audio operations on Raspberry Pi 4 (which isn't true on previous versions of Raspberry Pi). Filesystem activity does affect USB audio operation on Rasbian, even with an RT_PREEMPT kernel; but interestingly, filesystem activity has much less effect on UBS audio on Ubuntu 21.04, even on a plain PREEMPT kernel.
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There is some reason to beleive that there are outstanding issues with the Broadcom 2711 PCI Express bus drivers on Rasbian realtime kernels, but as of September 2021, this is still a research issue. If you are brave, there is strong annecdotal evidence that these issues arise when the Pi 4 PCI-express bus goes into and out of power-saving mode, which can be prevented by building a realtime kernel with all power-saving options disabled. But this is currently unconfirmed speculation. And building realtime kernels is well outside the scope of this document. (source: a youtube video on horrendously difficult bugs encountered while supporting RT_PREEMPT, by one of the RT_PREEMPT team members).
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For the meantime, for best results, log off from your Raspberry Pi, and use the web interface only.
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You may also want to watch out for temperature throttling of the CPUs. PiPedal displays the current CPU temperature in the bottom-right corner of the display. The system will reduce CPU speed in order prevent damage to the system if the CPU temperature goes above 70C (perhaps above 60C). The Pi 400 already has good heat sinks, so you shouldn't run into problems when running on a Pi 400. If you run into throttling problems on a Raspberry Pi 4, you may want to buy and install a heat sink (ridiculously cheap), or install a cooling fan. As a temporary work-around, you can orient the Raspberry Pi 4 board vertically, which can provide a real and meaningful reduction in CPU temperature.
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## Configuring PiPedal After Installation
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After PiPedal is installed, you can connect to the web interface as follows: via the mDNS address "http://pipedal.local" (on Windows, Mac, or iPhone web
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@@ -98,6 +75,30 @@ input onto the right channel of the USB audio inputs. So you probably want to co
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You can choose how to bind USB audio input and output channels (stereo, left only, right only) in the settings dialog. If you are using a Audio
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device that has more than two channels, you will be offered a list of channels to choose from instead.
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## Latency
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Note that Pipedal is not intended for use when logged in to Raspbian. Screen updates and heavy filesystem activity will cause audio dropouts. For best results, access PiPedal using the web interface remotely, through the Wi-Fi hotspot. Accessing the web interface via Wi-Fi has little or no effect on audio latency or dropouts.
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With a good USB audio device, PiPedal should be able to provide stable audio with 4ms (good), or 2ms (excellent) latency on a Raspberry Pi 4 when running on a stock PREEMPT kernel. Your actual results may vary.
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The current Linux kernel provides best latency on USB audio devices when they are configured with a sampling rate of 48kHz, and 3 buffers. Cheap USB audio devices (e.g. M-Audio M-Track Solo, available for less than $60) should be able to run without dropouts at 48kHz with 3x64 sample buffers. Most devices in this class use the same Burr-Brown chipset. Premium USB Audio devices should run stably at 48kHZ 3x32 sample buffers (about 2ms latency). I personally use the MOTU M2 USB audio adapter, which I highly recommmend -- stable, quiet, low-latency, great controls, and built like a tank).
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Make sure your system is fully updated, and that you are running with a kernel version of 5.10 or later, since version 5.10 of the Linux kernel provides improved support for class-compliant USB audio. The MOTU M2 (and many other USB audio adapters) will not work on versions of the kernel prior to 5.10.
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Prefer 64-bit operating systems to 32-bit operating systems. ARM processors execute 64-bit code about 40% faster than 32-bit code providing the same functionality.
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RT_PREEMPT realtime kernels (when available) are preferred but not required. (As if Februrary 2022, there aren't any good sources for latest versions of Ubuntu or Rasbian). PiPedal provides better (but not dramatically better) latency when running on a Raspbian Realtime kernel. Stock Raspbian provides PREEMPT real-time scheduling, but does not currently have all of the realtime patches, so interrupt latency is slightly more variable on stock Rasbian than on custom builds of Raspbian with RT_PREEMPT patches applied.
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The Ubuntu Studio installer will install a realtime kernel if there is one avialable. But -- at least for Ubuntu 21.04 -- there is no stock RT_PREEMPT kernel for ARM aarch64.
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On a Raspberry Pi 4 device, Wi-Fi, USB 2.0, USB 3.0 and SDCARD access are performed over separate buses (which is not true for previous versions of Raspberry Pi). It's therefore a good idea to ensure that your USB audio device is either the only device connected to the USB 2.0 ports, or the only device connected to the UBS 3.0 ports. There's no significant advantage to using USB 3.0 over USB 2.0 for USB audio. Network traffic does not seem to adversely affect USB audio operations on Raspberry Pi 4 (which isn't true on previous versions of Raspberry Pi). Filesystem activity does affect USB audio operation on Rasbian, even with an RT_PREEMPT kernel; but interestingly, filesystem activity has much less effect on UBS audio on Ubuntu 21.04, even on a plain PREEMPT kernel.
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There is some reason to beleive that there are outstanding issues with the Broadcom 2711 PCI Express bus drivers on Rasbian realtime kernels, but as of September 2021, this is still a research issue. If you are brave, there is strong annecdotal evidence that these issues arise when the Pi 4 PCI-express bus goes into and out of power-saving mode, which can be prevented by building a realtime kernel with all power-saving options disabled. But this is currently unconfirmed speculation. And building realtime kernels is well outside the scope of this document. (source: a youtube video on horrendously difficult bugs encountered while supporting RT_PREEMPT, by one of the RT_PREEMPT team members).
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For the meantime, for best results, log off from your Raspberry Pi, and use the web interface only.
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You may also want to watch out for temperature throttling of the CPUs. PiPedal displays the current CPU temperature in the bottom-right corner of the display. The system will reduce CPU speed in order prevent damage to the system if the CPU temperature goes above 70C (perhaps above 60C). The Pi 400 already has good heat sinks, so you shouldn't run into problems when running on a Pi 400. If you run into throttling problems on a Raspberry Pi 4, you may want to buy and install a heat sink (ridiculously cheap), or install a cooling fan. As a temporary work-around, you can orient the Raspberry Pi 4 board vertically, which can provide a real and meaningful reduction in CPU temperature.
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## Command Line Configuration of PiPedal
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The pipedalconfig program can be used to modify configuration of PiPedal from a shell command line. Run 'pipedalconfig --help' to view
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