Jitter v audio tehniki

[nek_do]

[ambasador]

@andrejS:

MSB je ena naj firm, ki je probleme rešila praktično u nulo. Stane kot res dober avto in nekaj pomeni v audiu. Lahko zdaj reči kako je vse irilevantno.
V za nas dosegljivem rangu ni pokošen travnik na voljo nego bolj zaraščen in grmičast.

Še en vidik. V preteklosti so na veliko dokazovali kako so ojačevalci popolni z meritvami THD z miljon nulami pred cifro pri polni moči. Poglejmo mi prvo desetinko vata - pa bomo hitro videli kje nastanejo razlike med ojačevalci!!! Vsa glasba se pretežno odvija na ekstremno nizkih nivojih, ambienca, barve instrumentov, odmevi, so tako drobni signali in se dogajajo zelo zelo zelo spodaj, okoli osi kjer se le rahlo premikajo gor in dol. Samo dinamični peak-i sekajo v višave in to samo momentalno.
Digi je perfekten za peak signale, pozabljeni pa so low low low level - tisti ki naredijo šmek. Tam pa je malo bitov na voljo.

Tudi ojačevalci so perfektni tako kot digitalaci - a puš-pul princip dela še vedno zgago pri prehodu okoli nule, še vedno ga single ended debelo nese kadar je vprašanje nizkih signalov.
_________________
Carpe Diem

[AndrejS]

Zivjo...

Jaz sem podal primer, da dobis obcutek za magnitudao. Jitter efekt na spektru pri -100dB, kaj bos z tem? Kot, da bi gledal novi avto, uspel pa opaziti celice mrtvih music na gumah.
Ce ze, lahko mirno recemo, da so elektricne motnje iz omrezja, ki se propagirajo po napravah znatno vecje od efekta jitterja. Ce ne gre po omrezju pa po zankah med napravami.

Karkoli ze, ko to pripeljes na zvocnik bo ravno on imel zadnjo besedo in ponavadi poje mimo v % skali.

Predojacevalnik za gramofon pride z vsemi mukami na -70dB pa temu reces rezulat kot se sika?
Meni se vidi, da tale MSB dela cisto vredu. Kaj ko je financno zavozen malo izven okvirjev okusa vecine ljudi.


LP, Andrej

[ambasador]

Mali popravek:
Phono predojačevalci ta dobri grejo na minus 85dB. VTL tudi na lampe to dosega (ca 9000 eur)!!
Integrirani ojačevalec Yamaha NS 2000 mu phono tudi leti na te cifre za manj kot 2jurja evrov.

Če upoštevaš da dobra long plejka omogoča 65 dB dinamike ob predpostavki da to posnetek tudi izkoristi - je popolnoma dovolj.

Tragedija je CD, ko zaradi jitterja ne dosega dinamike dobrih klasičnih gramofonov! Na papirju ima 96dB, dejansko v praksi pa šepa za longplejko. Sedaj to dosegajo samo najboljši - bolj so limit posnetki - šepajo pa za longplejko po informacijah. Bi moral čut CD7 da odkriješ dinamiko in se vprašaš - what a fu... smo poslušali 30 let.

Kako potem odobravati dokazovanje kako jitter ne vpliva, če ne dosega gramofona po dinamiki?? Vse kaj drugega kot opazovanje šibice iz 10 nadstropja.
Na, zdaj sem se spomnil tudi na zombije, ki lahko poslušajo pikosekunde med dvemi atomi zlata samo zato ker jim srce ne bije. #"!&# (ta je šifriran zelo sočen izraz).
_________________
Carpe Diem

[bojc08]

MSB je res na vrhu. Podoben dac, kot ga imajo oni mislim naret.
Vidim da je debata zašla, kaj še je slišno.
Efekt Jitter je zelo opazen tam spodaj, samo sistem mora bit dovolj sposoben, da reprodicira najtišje pasaže.
Upoštevat je potrebno tudi prostor, ki mora bit dovolj "gluh".

CD lahko ima majhen jitter in prekaša lp.
Samo to je teško doseči. Pred desetletjem ali več so takšne izdelovali.
Danes je pa hd v ospredju in se noben več ne ukvarja z to problematiko.
Danes ne izdelujejo več tako kvalitetne mehanizme oziroma pick-up, kot so jih včasih. To pa je pred pogoj za nizek jitter na cdp.

lp
Bojc

[bojc08]

Pa še nekaj, ker se toliko ponavlja.

NA CD NI POSNETIH 16 BITOV!!!

Ampak je stvar drugačna.
Teh 16 bitov, ki jih ponavljate izvira iz 14 plus 00.
Možno pa je posnet in predvajat 20 bitov!
Takšen je moj in dosega -110bB dinamike.
Takšen testni cd imam posnet.
To lahko vsakomur dokažem.
Veliko znancev pa za to že ve.

lp
Bojc
ps
potrebno je poznat malo pobližje zapis na cd-ju.

[mestn pu]

[vintagesonic]

[bojc08]

vintagesonic - to že vrabčki čivkajo. Vsi prepisujejo eno in isto teorijo.
Jaz poznam kako dekodita. Jaka od Hajdinjaka pa samo groba osnova.
Na cd-ju so sekvence 588bitov. En del je audio informacija.
To informacijo večina cdp pretvarja 8 to 14 bitov. In tudi igra 14 bitov. Temu primerna je dinamika. Od takšnih pa je res lp bojši.
Gre pa dekodirat drugače!
En od takih je HDCD, ali pa od Sony-ja SBM. Ti so drugače kodirani.

lp
Bojc

[bojc08]

lp
Bojc

[bojc08]

Ni mi za prevajat. Avtor je Professor David Joyner.

Reed-Solomon Codes and CD Encoding

In the digital age, all information whether it is sound, video, graphic, or text is 0?s and 1?s. Just as a picture is subject
to a scratch or sound is subject to noise, digital information is susceptible to error. For data to be usable, there is a
method of encoding information, which is organizing the 0?s and 1?s, such that errors can be corrected. Digital errors
are most often in the form of a 0 being received as a 1 or vice versa. One major applications of digital encoding is the
audio compact disk, or CD. CDs use a modified form of the Reed-Solomon code called the Cross Interleaved Reed-
Solomon Code, or CIRC. Before this can be explained in more detail, general error correcting codes will be introduced
along with the Reed-Solomon code.
Codes have three primary characteristics. These characteristics are the length, dimension, and minimum distance of a
code. The length of a code, or n, is the amount of bits (or information pieces) per codeword. The dimension, or k, is
the amount of actual information bits contained within each codeword. Minimum distance, or d, is the minimum
number of information differences between each codeword. A code is generally written as a [n,k,d] code. Coding
algorithms take information and translate it into a codeword. While there is theoretically any number of coding
algorithms, only a few have been implemented. This is because information must be able to be encoded and decoded
relatively quickly. Also, some codes may be able to correct a huge number of errors, but that means much of the
transmitted information is check bits. If check bits fill up a bandwidth, then the system is slowed down.
Minimum distance is the most important factor for determining a codes ability to correct errors. The following
example is the list of codewords for the [7,4,3] Hamming code:
[ 0 0 0 0 0 0 0 ], [ 0 1 0 1 0 1 0 ], [ 0 1 1 0 0 1 1 ], [ 0 1 1 1 1 0 0 ], [ 1 0 0 0 0 1 1 ], [ 1 0 0 1 1 0 0 ] [ 1 0 1 0 1 0 1 ],[ 0
0 0 1 1 1 1 ], [ 0 0 1 0 1 1 0 ]
[ 0 0 1 1 0 0 1 ], [ 0 1 0 0 1 0 1 ], [ 0 1 0 1 0 1 0 ], [ 0 1 1 0 0 1 1 ], [ 0 1 1 1 1 0 0 ], [ 1 0 0 0 0 1 1 ], [ 1 0 0 1 1 0 0 ], [
1 0 1 0 1 0 1 ]
[ 1 0 1 1 0 1 0 ], [ 1 1 0 0 1 1 0 ], [ 1 1 0 1 0 0 1 ], [ 1 1 1 0 0 0 0 ], [ 1 1 1 1 1 1 1 ]
Looking at the codewords, one sees that each differs from the others in at least three places. Using d, one determines
how many errors a code can correct. Let C be a Hamming Code with minimum distance d. Then C is [(d-1)/2] error
correcting. For example, the [7,4,3] Hamming code is 1 error correcting. If one received [1 1 0 1 1 1 0], he would
notice it is not a code word. However, it would quickly be corrected to [1 1 0 0 1 1 0]. If one received [1 1 1 1 1 1 0],
he would not know if it was [1 0 1 1 0 1 0] with errors in the 2nd and 5th bit, or [1 1 0 0 1 1 0] with errors in the 3rd
and 4th bit.
Reed-Solomon codes are linear block codes. They are often denoted RS(n,k) with s-bit symbols. The encoder takes k
data symbols and adds check symbols to make an n symbol codeword. Reed-Solomon codes correct up to t errors in a
codeword where 2t=n-k. For a symbol size s, the maximum codeword length (n) is n=2s-1. Because a Reed-Solomon codes correct byte errors, they can potentially correct many bit errors. A symbol error occurs when 1 to all bits in a
symbol are wrong. For example, RS(255,223) can correct 16 symbol errors. In the worst case, 16 bit errors may occur,
each in a separate symbol so that the decoder corrects 16 bit errors. In the best case, 16 complete symbol errors occur
so that the decoder corrects 16 x 8 bit errors. This makes a Reed-Solomon code very good at correcting large clusters
of errors.
Reed-Solomon codes are based on finite field arithmetic. Each codeword is generated using a generator polynomial.
All valid codewords are exactly divisible by the generator polynomial. The general form of the generator polynomial is
g(x)=(x-ai)(x-ai+1)?( x-ai+2t). The codeword is generated such that c(x)=g(x)i(x) where g(x) is the generator
polynomial, i(x) is the information block, and c(x) is a valid codeword.
A received codeword is a valid codeword, c(x), plus any errors, e(x). The Reed-Solomon decoder will identify the
position and magnitude of up to t errors and correct them. Any Reed-Solomon code has 2t syndromes that depend only
on errors. If the roots of the generator polynomial are substituted into r(x) where r(x)=c(x)+e(x), then the syndrome is
given.
The symbol error location is found by solving a simultaneous equation with t unknowns. Algorithms that do this take
advantage of the matrix structure of Reed-Solomon codes. Two steps are required to find the error location. First an
error locator polynomial is found. The two algorithms that find this special polynomial are the Berlekamp-Massey
algorithm and Euclid's algorithm. By analyzing the errors as the elements of a finite field, the Berlekamp-Massey
algorithm finds the shortest linear recurrence that will produce those elements. This algorithm usually leads to more
efficient software and hardware, but Euclid's algorithm is most often used because it is easier to implement. The roots
of the error location polynomial are found using the Chien search algorithm. Once the errors are located, the proper
symbol is found by solving the equation with t unknowns using the Forney algorithm.
Now that Reed-Solomon codes have been introduced, we will discuss how they are used to decode errors on a CD.
Audio CD's use the Red Book Standard, which was developed in 1980 by Sony and Philips to standardize how
information on a disk would be stored. The standard was printed in a red binder, hence its name. The encoding of an
audio CD is best understood by tracing audio information through the encoding process.
For sound to be "CD quality" it must be sampled at 44.1 kHz at 16 bits by two channels. This is a data rate of
1,411,200 bits/sec. 16 bits provides 65,536 (216) levels of sound information every 2.268x10-5 seconds. The audio data
is broken into frames of 6 16bit words/channel. This data is then converted to 24 8 bit (1 byte) words. Each frame
contains information for 1.36 ten thousandths of a second.
CD players uses Cross-Interleaved Reed-Solomon Coding, or CIRC. This begins by taking the 24 8 bit words in
encoding them in a RS(28,24) code. With 4 parity check symbols, it is 2 error correcting. The data is then interleaved.
This process distributes the information from this frame over 109 frames. This allows errors on a large portion of the
disk to be distributed over many small parts of the disk. This allows more errors to be corrected and prevents ruining
an entire block of information. This is demonstrated in the following simplistic example:
Three length three code words must be transmitted, [010][001][011]. Allow the code to be 1 error correcting. If the
information is not decoded successfully, something bad will happen.
If the first word is incorrectly received as [110], when the information is de-interleaved, the received words would be
[110][101][111]. Through error correction, the words would be decoded as [010][001][011]. If the data were not
interleaved, it would have been corrupted and unreadable.
After interleaving, the data is encoded in a RS(32,2 code. This is also a 2 error correcting code. The data is once
again interleaved with a new pattern. Interleaving works well for a CD because 1 frame is spread over 109 frames.
Each frame is 32 bytes long. Each frame corrects up to 4 symbol errors. 109 frames can correct 436 errors. With
interleaving, 13.625 frames can be corrected, where it would be impossible to do this without the interleaving.
The first code, RS(28,24) is called the C2 level of encoding. It corrects errors due to the physical condition of the CD
and the way that the CD was recorded. The RS(32,2 code is the C1 level and it corrects errors due to fingerprints
and scratches. This completes the encoding of the audio information, but control information must be added to the CD.
This is done by the addition of an 8 bit subcode to each frame. The subcode bits are designated P,Q,R,S,T,U,V,W.
Only the P and Q bits are used on audio CDs. Bits R through W are used for CD-Graphics like karaoke CDs. The P bit
is used to designate the start of a track. The Q bit provides timing information. The subcode bits from 98 frames are
collected to form 8 98 bit words. Because the subcode is located with the music, the exact location and time on the
disk is always known.
With the addition of a subcode block, the information is 33 bytes long. However, before it is encoded to the CD it must
be modified for use on a CD. The information undergoes eight to fourteen modulation, or EFM. With this, each 8 bit
word is assigned a 14 bit word from a ROM (read-only memory) dictionary. These words are designed to have a low
number of transitions from 0's to 1's. Also, this increases the minimum distance of the code therefore adding an extra
layer of error protection. Each 14 bit word is joined by 3 merging bits to aid in timing synchronization. Now the frame
is 561 bits long. Each frame is assigned a unique 24 bit synchronization pattern to proceed it along with three merging
bits. This makes one frame 588 bits long.
A block is made up of 98 frames. The rotational velocity of the CD changes so that the linear velocity over the laser is
a consistent 1.3 m/s. At this rate, 75 blocks are read per second. 1 block with 98 frames with 24 sound data bytes at 8
bits each by 75 blocks per second is a data rate of 1,411,200 bits/second. This reproduces the original sound
information rate. However, the actual data rate is 98 frames/block times 75 blocks/second by 488 bits/frame is
4,321,800 bits/second. CD-ROM format is slightly different in the amount of error correction it provides. The standard
data rate for a CD-ROM is 150 kB/sec, or 1,233,600 bits/second.
The CIRC code used on audio CDs can correct burst errors of up to 3500 bits (2.4mm) and can interpolate error bursts
of up to 12,000 bits (8.5 mm). Advances in technology in the past 20 years have lead to even more applications for CD
technology including DVDs. The error correction on a CD guarantees that high quality music can be enjoyed
consistently and reliably.

lp
Bojc

[vintagesonic]

[bojc08]

vintagesonic - že dolgo ločim kao 16 bitov. Igra samo 14 bitov.
Dosti sem delal z dekoderji in enkoderji in jih tudi meril! Analiziral!
Zgleda da vama z jakeco ni čisto jasno!
Za razliko od vaju se z tem praktično bavim.
Malo bolje si preberi pa ti bo jasno koliko bitov potrebuje za rekonstrukcijo.
Obstajajo pa še drugi načini dekodiranja.
Zmeraj moraš vzet celoten tekst, ne pa samo se piknit na kakšno besedo.

lp
Bojc

[vintagesonic]

[bojc08]

Ta tekst sem namerno prilepil za ljudi ki jih to zanima.
Tebi nebom nič več razlagal. Govoril sem o dejstvih, kar je resnica.
Zgleda da nisi nigdar meril teh stvari. Ravno v tem je hec.

Bistvo stvari pa je da moj cdp igra do -110dB!
Pa mi ti razloži na kak način.


lp
Bojc