DAC in MegaFlash ROM

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بواسطة Erikie

Champion (324)

صورة Erikie

27-07-2009, 09:19

I recently opened my MegaFlash rom and saw that it uses a different dac then original SCC carts from Konami. It only has a resistor array with 10 pins so it has 9 bits resolution and the original dac has 14 pins which gives you 11 bits resolution. Is this sufficient enough or can there be any sound difference in it?
Not that I noticed any difference myself though.

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بواسطة Leo

Paragon (1236)

صورة Leo

27-07-2009, 12:00

theorically 11 bits gives at most 66dB snr , and 9 bits 54dB, 12 dB is a lot.
A clean FM radio reception is about that ~50dB while a full digital top class DAC can give at best 100dB -120dB ( dreaming )
Regular TV set gives 60dB to 70dB SNR for analog reception ( at best ), for instance NICAM sound is 14 bits compressed to
11bits and sounds very clear.

If the konami SCC is really limited by the bit width then there is a real hearble difference, now the question
is : is there any other source of noise limiting SNR apart from bit quantification ? i guess so , it could be the
internal msx circuitry.

I am used ( a bit ) to work with all this THD + Noise in digital electronics and audio measurements , i would not
say the whole chain SCC cart + MSX + TV loudspeakers sound at 66dB SNR , in my opinion the most hearable
defect of SCC is distorsion.
All in all i think there is no hearable difference ( that does not mean there measurable difference ), though i
would not go any lower than 9 bits ...

بواسطة Guillian

Prophet (3516)

صورة Guillian

27-07-2009, 13:19

The DAC used in MegaFlashROM SCC is 8bit (10 pins = 8bit + GND + out) It is almost impossible to find a 11bit DAC resistor array network.
I think there is no noticeable difference.

بواسطة Erikie

Champion (324)

صورة Erikie

27-07-2009, 13:23

Isn't it cheaper to use small SMD resistors to make your own DAC? I made one on the SD device Leo has.
Costs for this is pretty low as you only need a bunch of 1k and .5K resistors which costs allmost nothing Smile

بواسطة Bastiaan

Champion (333)

صورة Bastiaan

27-07-2009, 13:47

Or for some pics of non smd, see also:

http://sharksym.egloos.com/4005510

بواسطة Guillian

Prophet (3516)

صورة Guillian

27-07-2009, 14:12

As I posted in http://www.msx.org/forumtopic8877p15.html
I asked to Kazuhiro Tsujikawa about the SCC DAC and he told me that making a 11bit DAC with standard resistors is not a good idea, since you need a 0.05% accuracy (standard resistors accuracy is 1-5%). So problably an 8bit DAC (0.4%) is enough.

Moreover, soldering resistors for a DAC is fine for a few units. But if you have to make 100 or 200 SCC cartridges by hand... then it is better an array network ^^!

بواسطة sunrise

Paragon (1091)

صورة sunrise

27-07-2009, 14:39

Why inventing the wheel again and again. Manuel did already the job ! And even a better version comes up

بواسطة Bastiaan

Champion (333)

صورة Bastiaan

27-07-2009, 14:57

@ Guillian: I don't get it. Please explain why the accuracy should be 0.05% or 0.4%.

I cannot hear any difference between my standard resistor DAC on my SD/MMC interface, and my original Konami.

بواسطة Leo

Paragon (1236)

صورة Leo

27-07-2009, 15:01

8 bis might be enough , but just one observation :
If the resistor dont have the good value , that means the DAC is not as linear as wanted and this may end up with distorsion.
If there are less bit (8 instead of 11, for isntance) you bring noise of quantification , which is not exactly the same.

in other way :
if you approximate a 11 bit sample with 8 bits , sometime you are doing an error of 1 bit some other time an error or 2 bit and so on, it varies = noise.
When a resistor has to not the good value by max 0.5% the error is always the same for that given bit.

that percentage i think comes from :
with 8 bits you have 256 levels so each level is 1/256th step = 0.0039... = 0.4%
with 11 bits you have one step = 1/2048 = 0.000488 = 0.05%

this means that least significant can weight more than the bit before ! but this is not noise it is distorsion !

More over , i believe the dacs are not made with resistors of different values ( each being double of next stage)
but are made with resistor in // which divide their respective values ( and relative error also ! ) .

بواسطة hap

Paragon (2042)

صورة hap

27-07-2009, 17:15

Does this DAC apply to the output volume too? According to my measurements, normal SCC vs MegaFlashROM SCC volume is around factor 3:2.

بواسطة SaebaMSX

Hero (533)

صورة SaebaMSX

27-07-2009, 18:08

Hey, my MegaFlashROM SCC was made from an original Contra, so it should have the same factor! Tongue

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