Electrical Engineering senior design (vinyl -> .flac)

[champyg]

Hi all. I've been searching for some solid electronic hobbyist forums that could lend me some advice for my senior design project, and I found myself here. I'll probably end up using this thread as a build log as well should it get enough attention. So here goes...

My group is making a standalone device that will read an audio signal from a vinyl via stylus, digitize it, and send to a PC where it will then be converted to a hi-fidelity audio format, most likely .flac. I realize we're not exactly reinventing the wheel here, but it seemed like a fun project and our professor approved it, so why not. A few extra tasks we came up with to make it sr. design worthy are:

1. DIY deck assembly - this means we'll be making our own motor control system w/ feedback
   
   
2. Line-in level LED array - This will serve as a crude line-in level meter so we can see if our analog signal is too low or clipping
   
   
3. DIY amplification/filtering of analog signal - this is the part I am most concerned about. I know audio signal are very sensitive to noise and require careful consideration to accurately recreate. Having no experience in this area, I'm sure most of my questions will stem from this.

Currently, we're still in the research phase, but we really need to buy parts in the next week or two so we can start playing around with something. My assignment right now is figuring out what is needed to amplify/attenuate/filter the analog signal before we digitize it. For now, our plan is to use an MSP430 (not sure exactly what model, but it's being loaned by my professor) to handle the A to D function. Scoping up to the stylus of our MONO record player shows a voltage signal that seems to average around 5-7V w/ spikes around 9-10V. I was surprised to see a voltage this high just from the stylus. It looks like I'll need to attenuate instead of amplify to bring it to suitable levels. And that's basically as far as we are currently. If you read all the way through this, I appreciate all the interest and any help you can give.

tl;dr: Though I scoured the spec sheets, I couldn't seem to find what input voltage is needed for ADC on a Ti MSP430 board. I'll get the exact model asap. Also, what are my options as far as audio signal attenuation?

Thanks

 

[Nigel Goodwin]

Scoping up to the stylus of our MONO record player shows a voltage signal that seems to average around 5-7V w/ spikes around 9-10V. I was surprised to see a voltage this high just from the stylus. It looks like I'll need to attenuate instead of amplify to bring it to suitable levels.

The output from a PU is nothing like that - where are you measuring?, and what kind of PU is it?. EXTREMELY high output crystal PU's can output around 1V, these were specifically designed to feed single pentode valve amplifiers in cheap record players. Normal decent quality moving coil PU's output a few 10's of mV, and require an RIAA preamp, of which there are countless designs on the net.

Everything you need is pretty simple, turntable, PU, PU preamp (RIAA), USB sound 'card' - job done, ten minutes

 

[champyg]

I believe our pickup is a moving coil or magnet type. I tried looking for pictures, but didn't come up with any that would help me distinguish. I can't get hands-on anything right now to show pictures, but I'll take some later on today. Also that measurement was taken at the solder pads for the stylus. I can scope up again later today and make sure I wasn't using the scope incorrectly.

A few other questions. Are you familiar with the MSP430 family? Will it suffice for this project? Also, will I need to get a dedicated sound card to handle the encoding? My creative in my desktop just blew up, but I can order a new card if onboard isn't up to par.

 

[Nigel Goodwin]

I believe our pickup is a moving coil or magnet type. I tried looking for pictures, but didn't come up with any that would help me distinguish. I can't get hands-on anything right now to show pictures, but I'll take some later on today. Also that measurement was taken at the solder pads for the stylus. I can scope up again later today and make sure I wasn't using the scope incorrectly.

A few other questions. Are you familiar with the MSP430 family? Will it suffice for this project? Also, will I need to get a dedicated sound card to handle the encoding? My creative in my desktop just blew up, but I can order a new card if onboard isn't up to par.

Isn't it a Texas Instruments microcontroller?, what are you planning using it for? - the PU preamp can simply feed in the computers sound card input, or use an external USB sound card.

 

[champyg]

The microcontroller will be handle all the A to D as well as the motor control and led array logic. Since this is a senior design project, we need something a little more robust that just outputting the signal to a sound card.

Also, I scoped up to the signal wire again and even isolated the audio signal entirely from the PCB. Still getting an output that averages 4-5V. Seems like newer records have a lower output. The turntable is a Califone 1030av and it uses an 89t cartridge.

 

[Nigel Goodwin]

The microcontroller will be handle all the A to D as well as the motor control and led array logic. Since this is a senior design project, we need something a little more robust that just outputting the signal to a sound card.

The mechanics of building a turntable should keep you busy, and the servo systems if you're going that way.

Also, I scoped up to the signal wire again and even isolated the audio signal entirely from the PCB. Still getting an output that averages 4-5V. Seems like newer records have a lower output. The turntable is a Califone 1030av and it uses an 89t cartridge.

You're doing something terribly wrong if you're reading that high off a PU, of any type - and as you expressed an interst in high quality you should be looking at magnetic ones, not cheap ceramic/crystal.