Featured Questions & Answers
- What guidelines are there for microphone placement when using VocalFusion solutions?
We recommend the following general guidelines regarding the placement and porting of microphones:
- Microphones should be placed as far away from speakers as possible.
- Microphones should be isolated as much as possible from system vibrations. This may mean mounting microphones on separate PCBs or flex to provide maximum isolation.
- The microphone port should be well sealed to the enclosure to ensure internal product noise and internal speaker energy is not coupled into the microphones.
- Speakers should be acoustically isolated from the chassis to reduce coupling into the microphones.
- The space between the microphone and the external enclosure should be designed to avoid Helmholtz resonance within the band of interest.
The effect of the enclosure should be checked by comparing frequency response of the system with and without the enclosure. A seal test shoud be made (where you compare the frequency response with the microphone ports open with the response when the ports are acoustically sealed) to help understand internal leakages and interferences.
- How many channels of PDM microphone can XMOS support in one device?
The XUF216 device has two tiles - each with a 500MHz processor which can support dual issue (2 instructions per clock). Each tile has up to 8 logical cores which share this processing. So peak throughput is 500 * 2 * 2 = 2000MIPS for the XUF216 chip. These are "will not exceed" numbers. More useful info is that a well written FIR (like those used in PDM decimators) can achieve around 2 cycles per tap.
If you connect 32 MEMS PDM microphones to a XUF216, you will have just enough MIPS to get samples on to TDM and about 62.5 MIPS for DSP. However you could use an XUF232 and you would have two whole tiles free.
The PDM clocks are synchronised. Each of the channels are precisely synchronised right through the decimation filters to PCM. This is needed for DoA, beamforming and other algorithms that require subsample precision.
- What is the PDM to PCM latency value for output sample rates?
The group delay of the default filters is 18 output clock cycles.
This can be shortened by either using a minimum phase FIR as the final stage decimation FIR and/or by reducing the number of taps on the final stage decimation FIR.
This is true across all supported sample rates.
- Which driver do I need to use the xCORE Array Microphone - XK-USB-MIC-UF216?
To run the kit as a simple seven channel USB microphone, download the USB Audio 2.0 Software
and then build the 'app_usb_aud_mic_array' app.
Select the '2i8o2' or '1i8o2' build configuration depending on whether you would like USB Audio Class 2.0 or 1.0.
- Does XMOS provide USB drivers for third party USB products?
XMOS does not provide drivers for third party USB products, even if they use XMOS USB chipsets internally.
If you need a driver for your USB device, please contact your supplier who will be able to provide you with a drivers for their product.