ISD4003 Noise Problem

[Agent36]

Hi All,

As there are quite a few posts on here refering to the ISD4002/3 chips, some one must have had the same problem.

I am getting alot of hiss when the chip is in playback mode. I load clean audio onto the chip, when I play it back I get background hiss along with the audio.
I have been through most of the chipcorder application notes and carried out their tips. Such as, seperate digital and audio supplies, large ground planes, plenty of decoupling caps etc.

The components on the board are the recomended values in the isd4003 data sheets.
I am using a lm386 audio opamp which is one of their recommended audio amps into an 8ohm speaker. (I have the gain set to 20) I have built the amp circuit as per the lm386 data sheet, still cant get rid of the hiss.

I have also modified the pic software to remove the power up pops during the record mode.

The only other option I can see is to filter the hiss out. I dont have access to a scope, so I cant view the wave form. If it helps the hiss is like the noise you would get from a tape recording played very loud.

Any suggestions would be much appreciated.

Thanks for your time.

Nick

[mister_e]

Yeah those ISD s u c k s a little bit. make sure you record at maximum amplitude (just befoe clipping) it should solve one part of the problem.

Now the Hiss, it's just a matter of adding a low pass filter... but it will alter the high frequencies... as long as you can deal with it, it shouldn't be much of a problem.

[Agent36]

Hi Mister-e,
Thanks for the reply, can you suggest some values for the low pass filter? Filters are a big subject and I dont have any experience in that area. I did try googling for some info before i started this thread.

Kind regards

Nick

[mister_e]

bah, to make things simple, try few capacitor values in parallel with your LM386 input first. Try a range on 0.01-0.1 uF. You may find something interesting is this range.

Low-Pass between 6Khz-10Khz will remove most hiss noises... and more ;)

A scope will be useless in this case, you need a Spectrum analyser (hardware or software)

[JoelMurphy]

I've played around with the ISD series a bit for a client. They are amazing chips, but the audio quality is not so good if you get it right out of the can. Filtering is crucial. A good source for that and more [so much more] is Paul Sherz's book: Practical Electronics for Inventors. Widely available. But note, there are some amazingly emberassing typos.

[Agent36]

Hi,
Thanks for the reply. I will try and source that book, it sounds like I can use it for other projects and problems.
I am still having problems with noise, do you have any info on the filter part of your isd4000 designs. i.e what type of filter and values, basicly anything will help. I solved every other problem i've had on this project, it does every thing I want.

thanks for your time

Nick

Found that book!! just ordered it, thanks...

[Agent36]

Hi,
Is there anyone that has access to a Spectrum analyser? I still have noise issues with the isd4002. If possible can someone look at the wav file using a Spectrum analyser then tell me what frequencys I need to filter. I am unable to upload the file. If you can pm me with your email i'll email the wav file.

kind regards
Nick

[mister_e]

Yup i have few of those here. You could still try some Software solutions too.

try TrueRTA and Visual Analyser, to me, they look good for some freeware.

Visual Analyser
http://www.sillanumsoft.com/

TrueRTA
http://www.trueaudio.com/

I didn't used them, i use Smaart Pro, SpectraLab and/or a good ol' Klark-Teknik DN60 RTA for my studio or on the field PA rig tune-up.

[Agent36]

Thanks for the links, I have downloaded the visual prog, going to try it later this week when I have time.

Regards
Nick