Analogic vs Digital

[Shibata]

Exact. Orice filtru digital introduce acel riplu (care arata chiar mai urat pe semnal dreptunghiular).

Da, vizualizat personal rezultatul pe un osciloscop analogic (amarasteanul ala din dotare) acum cativa ani, cand m-a preocupat problema. Pot repeta experimentul la nevoie, dar va trebui sa fac poze dupa CRT, ca n-am cum sa transfer altfel datele in computer.

Dar gasesti si pe Stereophile niste masuratori, parca la un Audionote...

[Andi]

Playerele din prima sau a doua generatie nu erau tot nos? Din cate stiu parca nu faceau oversampling...

[Shibata]

Sincer? Habar n-am. Dar stiu ca DAC-urile alea vechi (de la Philips) mergeau pe mai putin de 16 biti, ceea ce genera probleme mult mai mari. Eu am sapat putin in TDA1541 si numai in chipsetul facut pentru el (adica SAA7210 + SAA7220). Nu ma intreba de rest.

[Andi]

E interesant ce spun si cei de la Philips pe subiect. Par sa fie principalii vinovati de introducerea filtrului digital

"Oversampling

With the use of oversampling technology - which allowed the 16-bit Red Book standard to be met with the 14 bit digital-to-analogue converters that were available when the system was introduced - a very high sound quality was obtained. The only difference, in the Philips players, is that this is temporarily multiplied by a factor of four. As well as allowing 16 bit-equivalent performance, this also offers additional benefits. For example it gives a higher signal-to-noise ratio and dynamic range. And instead of requiring a steep analogue output filter, it allows a relatively simple analogue filter to be used to suppress the remaining interference signals. Because even digital filters still allow some interference signals to pass through, which is why every player with digital filters also has an analogue output filter. The higher the degree of oversampling, the more effectively that a digital filter can suppress the interference signals, and the simpler the analogue output filter can be. Which in turn benefits the final reproduction quality. It’s also a positive factor that the use of a digital filter has no audible effects, while a steep analogue filter causes phase changes, which affect the overall reproduction quality.

However Philips’ oversampling technology, originally born out of the necessity to use the early 14 bit D/A converters, and dismissed as a 'technical joke’ by other manufacturers who believed that a true 16 bit D/A converter followed by a steep analogue filter was the only way to go, was quickly embraced by most manufacturers of CD players. Because it meant there was no need to use highly complex analogue filters, while at the same time it allowed the often serious non-linearities of the D/A converters that were available at the time to be concealed. "

[Andi]

Apropos, din cele de mai sus eu inteleg ca playerele din primele generatii, altele decat Philips, cu dacuri pe 16 bit erau nos.

[Shibata]

Depinde de DAC, depinde de chipset. Eu pot sa-ti spun numai ca oversamplingul in cazul CDP-urilor clasice cu TDA1541 il face SAA7220. De aici si moddingul NOS care presupune bypasarea SAA7220. Practic insa, se pot construi CD-playere si fara SAA7220 (adica numai cu 7210-le), si chiar cred ca au fost construite. Da, alea sunt NOS.

PS: acum, SAA7220 mai face si altceva in afara de oversampling si anume interpolarea semnalului audio in cazul aparitiei unor erori de citire catastrofale, care nu pot fi corectate de SAA7210. E un semnal, se numeste EFAB, prin care 7210 ii spune lui 7220 ca datele pe care i le furnizeaza sunt eronate si sa refaca semnalul prin interpolare. Practic insa (din cate am studiat eu de ex. punand un monostabil cu un LED pe EFAB, erorile alea sunt extrem de rare. M-am chinuit o zi intreaga sa zgarii un CD astfel incat sa vad aprinzandu-se LED-ul de EFAB. Asta a fost acum cativa ani insa...

Philips are chiar un CD special (pt. service) care genereaza astfel de erori la citire. Daca si p'asta il are Brainsmartman in dotare... eu ma impusc!!!