Originally Posted by piper91765

Hello all,

I have a senior project that I need to get started on and I was looking for some guidance. My goal is to record a guitar solo for later playback using a µController. I need to create a circuit that has the following functionalities:

1) Record mid to high quality audio using a µController, audio recording IC, or a combination of the two.
2) IC/uC must be able to store 1-2 minutes with a sampling rate of 44.1k.

Assuming mono, and 16 bit (or 10 bit if using most common PIC's), you will require just over 5 megabytes per minute.

3) Store this audio using either internal memory or on an external SD card.

PIC's have a few tens, or at most hundreds, of bytes of 'RAM', so that's not going to happen

An SD card is a better possibility, but I'm not sure how fast you can write to them?.

4) Have the ability to playback the audio on demand via control of a uC.

That's no problem at all, you should be able to read an SD card that fast easy enough and output via a D2A.

Obviously you MUST have anti-aliasing filters on both input and output.

Here are some designs that might help you out:
120 Second Voice Recorder
Simple Voice Recorder

Give more details on your project requirements and restrictions - such as type of microcontroller, budget, etc...

It's similar to the links posted, a voice quality recorder chip - like I said above, you need around 5 megabytes to store 120 seconds of mono 16 bit audio at CD quality.

i did such a project with a SD card and a dsPIC30F without any other ICs. It could do 48Khz without a problem. But i didn't make it record also.

Yeah i have a little for you

Code that interfaces a SD or CF memory card or a IDE harddrive and even handles a FAT12 FAT16 or FAT32 filesystem
http://www.microchip.com/stellent/id...pnote=en532040

Information about the WAV file format
http://ccrma.stanford.edu/courses/42...ts/WaveFormat/


As for the DAC i just used a PWM at a very high frequency and used a resistor and capacitor filter on it, It seams to produce a excellent sound ( better than a cheap PC sound card ) Or one of the dsPIC33F chips has a built in audio DAC with its own buffer.

Originally Posted by piper91765

I hear what you're saying Nigel, I'm leaning toward the SD card idea because the 120 sec chip has a max sample rate of 8.1 k samples/sec (that's telephone quality)

I calculate 5.05 MB per min:
44,100 (samples/sec) *60 (sec/min) *16 (Bits/sample) * 1/8 (byte/bit) * 1/1,048,576 (MB/ bytes)= 5.047 MB (using unit conversion)

Sorry, refer to my earlier post, 5 megabytes per minute.

Let me know if any of that is incorrect. Are you dividing by two due to the mono? That is to say 22,050 samples per channel per second?

No, the 44.1KHz is per channel, I incorrectly typed the time for one minute only.

Originally Posted by piper91765

...It basically says that a 32x SD card can be written to at a speed of 4.8MB/s, i'm not sure if a PIC can write to the card that fast, but the card seems to have the capability of accepting high speeds.
Also, i'm worried about programming. I am assuming that I should try and store this data in a relatively known format such as WAV, is anyone aware of a sample code that I could look at to help me program this pic to store the audio in WAV?

In SD 4-bit mode, that speed is probably doable. In SPI single-bit mode, you'd be lucky to hit 1 MB/sec. The 25 MHz max clock speed and the cards internal latency will reduce the transfer speed considerably.

The good news is that you only need to transfer 88.2 KB/sec. The SD card could do that in stereo. Or quad.

16 bit wave files (uncompressed) are incredibly easy to process. You just have to read the data from the SD card into a RAM buffer, read out two bytes 44100 times a second, popping the data into a 16 bit DAC. The output of the DAC goes into an audio amplifier, and Voila! CD quality sound.

Use an interrupt service routine to keep the DAC busy. The ISR can flag the main program when the RAM buffer needs attention.

The nice thing about the SD card's SPI mode is that the clock can be slowed down or shut down at any time. The ISR can interrupt the program at ANY point, and the SD card doesn't even care!

SD cards are fast no drought even with 25Mbit SPI. But you are going to need a MCU with a lot of ram. I recommend you have an at lest 1KB of buffer between the card and the DAC. That is because the card is read in 512B chunks, reading one chunk is fast. But when it gets to the end of it it needs to go look back in the FAT16 table where the next chunk it then just to that chunk and read it.This takes some time you you need a big enough buffer so it dosent get drained completely when it dose that.

Also take a look at the dsPIC33F micronctroler family. They have a 40MIPS 16bit CPU with a DSP endgine that helps it with math. Also it has a preperial inside that moves data between ram and other preperials without the CPU doing it.Also one other sweet thing is that some have a built in stereo audio DAC that has its small buffer in it.This means you only interupt when your buffer needs to be filled That means a LOT less interupts and saveing a lot of the CPU time so you have lots of room for other stuff. (These MCUs are esaly fast enough to do things like digital tone controll and such effects along playing it)