Multiple of 44.1kHz Xtals?

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Postby Lawnboy » Sat Dec 03, 2011 6:27 am

i thought it looked too much like a ceramic resonator myself, but dont those usually have 3 leads? i assumed it was a crystal from the marking on the board. Domonic Beesley had some issues with cd player stability when he was designing NBSC. http://authorityfile.co.uk/NBSC/article ... struct-5.1.
i wonder if the newer cd players -the ones with electronic skip protection- may be a bit better since they save parts of the data stream to memory.
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Postby Steve Anderson » Sat Dec 03, 2011 1:50 pm

Lawnboy wrote:i thought it looked too much like a ceramic resonator myself, but dont those usually have 3 leads?


Often, yes. The three-lead versions simply have the usual capacitors built-in, or they're for use as a poor-mans IF filter, typically for AM radios, sometimes FM too.

Most micros can use either a crystal or a ceramic resonator as their clock where frequency stability isn't important, e.g. controlling a washing machine. But most of the more recent micros have an internal RC oscillator which can be used instead of an external crystal, typically ±2% frequency accuracy at room temperature.

This frees up a couple of pins on the micro so they can be used as general purpose I/O.

Speaking of washing machines, the version here has programmable spin speeds up to 1000RPM....anyone thought of using one? You surely would get nice clean pictures!

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Postby Klaas Robers » Mon Dec 05, 2011 6:20 am

Steve, the point is that a CD doesn't contain only aidio bytes. There is also something called subcode and that adds to the byte count. So the X-tal oscillator runs quite high. I have a professional CD-mechanism that has an X-tal of 16,9344 MHz. That happens to be 384 x the 44,1 kHz. The number 384 = 128 x 3.
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Postby Steve Anderson » Mon Dec 05, 2011 2:01 pm

Hi Klaas,

Yes, we eventually got to the bottom of this. What I was looking for was the audio sampling rate such that I produce a 44.1khz .wav file which is easier to 'package' via software than a 48kHz version for those wanting to make an audio CD. Therefore eliminating that extra chore of sample-rate conversion...which may be automatically done by the burning software anyway.

I was aware of the sub-code and the error-correction data, but that's not required in a .wav file.

What I'm putting together will actually produce 48kHz .wav files as apart from the audio CD nothing else uses 44.1kHz...as far as I'm aware. But I just wanted to be able to offer the option of 44.1kHz for those that wished it.

Thanks,

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Postby Lawnboy » Mon Dec 05, 2011 2:54 pm

Steve, a little OT here: is it possible to "drag and drop" a crystal to replace the resonator in my cd player? when i eventually finish NBSC i will need a very accurate source and would rather not be tethered to my desktop.
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Postby Steve Anderson » Mon Dec 05, 2011 3:26 pm

Lawnboy wrote:... is it possible to "drag and drop" a crystal to replace the resonator in my cd player?


Not really OT, it is a related question. Of the same frequency, I would think so. I've not used ceramic resonators before, but the data for every PIC micro-controller shows the use of either a crystal or a ceramic resonator with the exact same arrangement.

They do hint that with a resonator there may be a resistor in parallel which if it exists will need to be removed for a 'crystal upgrade'. Although it's most unlikely to be a PIC in this case, I would expect the oscillator to be much the same....simply a Schmitt-trigger inverter or similar as per PICs.

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Postby Klaas Robers » Mon Dec 05, 2011 6:33 pm

I had insufficient time yesterday......
The precision of the frequency for a musical instrument like a CD-player is not that stringent. The frequency interval bedtween two half tones, so between C and CIS is the twelfth root of 2, which is 1.056, = 5.6%. This interval is divided in 100, called "cent". One cent thus is about 0.6 promille, 6 E-4.

Nobody hears a deviation of one cent. People with well trained ears hear 5 cent difference between two tones, so first tone 1, then tone 2. That is a frequency difference of 3 promille.

However, that has nothing to do with hearing the pitch of one note in the abcence of a reference. If a CD player has a deviating 16 MHz, all notes shift upward or downward and that can be e few percents without notice. So a ceramic resonator is under all circumstances more than adequate, and at all in a portable player.
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Postby AncientBrit » Mon Dec 05, 2011 7:15 pm

@Steve,

Have you considered using a PIC driven from an LC oscillator instead of a Xtal?

In the past I have used an HC132 feeding an LC pi network via a 220R, with the pi output fed back to the input of the HC132.
I found that the frequency stability is quite good and besides the square wave output of the Schmitt, a bonus is a near perfect sine wave from the output of the pi network (ie Schmitt input).

Locking the PIC to this square wave and choosing a suitable software divisor should give you the required 44.1kHz.

Regards,

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Postby Steve Anderson » Mon Dec 05, 2011 8:10 pm

I'm no expert in this field, but if a backing track (this is getting off-topic, but) were played back on a CD player (or whatever) and it was 0.5% out due to the use of a ceramic resonator and you wanted to overlay some additional instruments, then a re-tune may be needed. No big deal with stringed instruments or even electronic ones, Korgs for example. But a pain for a strung piano or a pipe organ. I repeat I'm no expert.

I admit, anyone using a resonator-clocked CD player as a mix-down/recording source is potentially asking for trouble...the lesson...use professional gear.

I realize that manufactures want to keep their costs down, but the difference in cost between a crystal and a ceramic resonator is insignificant in the grand scheme of things. For just simple playback of music and for Joe Public, I agree, a resonator is probably good enough. Bloody accountants again!

********************************************************

I wanted to use a crystal as it's accurate and temperature stable and just it and two small capacitors and you're done. I've ordered some from Farnell at 11.2896MHz which using the x4 PLL in the PIC will also result in an instruction cycle frequency of also 11.2896MHz. 256 cycles per audio sample. Ditto 12.288MHz for 48.00kHz.

I'm using the newer range of PICs at 3.3V which are quite happy up to 64MHz, initially a PIC18F46K20. At 3.3V it also interfaces with SD cards perfectly and using a MAX3232 allows RS232 terminal interface during development.

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Postby gary » Mon Dec 05, 2011 8:28 pm

Steve Anderson wrote:I admit, anyone using a resonator-clocked CD player as a mix-down/recording source is potentially asking for trouble..
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Anyone doing analogue mix-downs or recordings from a CD deserve what they get! ;-)
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