Optical drive

[alfe]

Often I read that the software used for ripping is more important than the drive.

I will skip the comment about the 100% accuracy and that after ripping it sound exactly like the master.

In this series of articles about optical drives I will start with the brushless DC spindle motor and the Jitter created by offset drift of the Hall effect sensors. The impact started to be tackled in 2000 and some solutions were proposed: http://www.dtic.mil/dtic/tr/fulltext/u2/p011838.pdf , few years later the research is still on-going: http://www.chalcogen.ro/883_Paun.pdf.

The power supply do have an impact on this phenomena, this document from Honeywell may help to understand: http://sensing.honeywell.com/index.php?ci_id=47847

So when you are ripping Jitter can be created depending on the quality of the spindle motor that your drive is using, the quality of power supply and the compensation that is put in place.

 

If you find that of interest the next article will be about optics and laser diode and how they can affect the sample amplitude.

 

 

[Jud]

Wonderful to see you back here, my friend. I will read the articles with interest.

[alfe]

Wonderful to see you back here, my friend. I will read the articles with interest.

Thank you Jud

[YashN]

Subscribed.

[alfe]

The Objective Lens

 

« Presently disc drive use lenses for focusing the laser beam to a diffraction-limited spot. These lenses consist of two aspheric surfaces, have fairly large numerical apertures, and are essentially free from aberrations. The numerical aperture of a lens is defined as NA = sin teta, where teta is the half-angle subtended by the focused cone of light at its apex. A 0.5 NA lens, for example, will have a focused cone whose full angle is 60°. The diameter of the focused spot is of the order of lambda₀/NA, where lambda₀ is the vacuum wavelength of the laser beam. It is thus clear that higher numerical apertures are desirable if smaller spots (and therefore higher recording densities) are to be attained. Unfortunately, the depth of focus of an objective lens is proportional lambda₀/NA², which means that the higher the NA, the smaller will be the depth of focus. It thus becomes difficult to work with high NA lenses and maintain focus with the desired accuracy in an optical disc drive.

But a small depth of focus is not the main reason why the optical drives operate at moderate numerical apertures. The more important reason has to do with the fact that the laser beam is almost invariably focused onto the storage medium through the disc substrate. The disc substrate, being a slab of plastic, has a thickness of 1.2 mm. When a beam of light is focused through such a substrate it will develop an aberration, known as coma, as soon as the substrate becomes tilted relative to the optical axis of the objective lens. Even a 1° tilt produces unacceptably large values of coma in practice. The magnitude of coma is proportional to NA³, and therefore, higher NA lenses exhibit more sensitivity to disc tilt. Another aberration, caused by the variability of the substrate's thickness from disc to disc, is spherical aberration. This aberration, which scales with the fourth power of NA, is another limiting factor for the numerical aperture.this lead to develop a servo mechanisms whereby the tilt and thickness variations of the disc are automatically sensed and corrected using a Gaussian focus as a reference point. »

This theory about solving spherical aberrations is questionable because the limit of the scaling laws for correction are easily reached :

http://homepage.tudelft.nl/99s1c/pdfs%20mypapers/ao_2005_1.pdf

 

Spherical aberrations can creates small phase shift with a direct incidence on sample amplitude.

 

Next article laser diodes.

 

 

[sandyk]

Welcome back my friend !

 

And for those who don't know already, Alfe has several patents, and is the guy who designed the LG GGW H20L BluRay writer that I use, so he obviously has a vast depth of knowledge in this area.

 

Kind regards

Alex

[AlainGr]

Welcome back Alfe I also miss your humor

 

Kind regards,

[elcorso]

Welcome back my friend !

 

And for those who don't know already, Alfe has several patents, and is the guy who designed the LG GGW H20L BluRay writer that I use, so he obviously has a vast depth of knowledge in this area.

 

Kind regards

Alex

 

Good to know Alfe is one of the jitter creators... moments ago I was looking for whom to blame

 

Welcome Alfe !

 

Roch

[alfe]

Thanks everyone

[Norton]

Thanks everyone

 

Hi alfe

 

A while back I followed up on your recommendation of a Plextor LB950UE as a good quality option for occasional CD replay.

 

Via the Plextor I really quite like CD replay via HQP or my Bryston BDP and wondered if you felt the Plextor was about as good as it gets or if there is a current external drive out there to better it for ultimate SQ?

 

Thanks again

[alfe]

Laser diode:

A general characteristic of laser diodes is their uniqueness. Although laser diodes are usually manufactured in great numbers in one process, every laser diode is slightly different in comparison to another unit of the same batch. As a consequence, the emission properties such as central wave- length, shortest pulse width or achievable output power does slightly vary from diode to diode.

CD use a pulsed laser which is a system which will emit light in the form of optical pulses, rather than a continuous wave (CW). There are numerous methods to achieve laser pulsing, but the end result follows the same principles.A pulsed laser periodically emits pulses of energy in an ultra short time duration.The duration, or pulse width for laser diodes can range from nanoseconds to picoseconds. The average power of a pulsed laser is defined by the amount of energy released over the period of the cycle which equal Energy × frequency.

Which bring us back to timing (jitter) (sorry Roch ) and Power (you are often right Alex ).

 

 

 

 

Noise and Jitter: http://www.informit.com/articles/article.aspx?p=1087655&seqNum=2

 

 

Infos about CD and LED : http://www.ld-didactic.de/documents/de-DE/EXP/PHO/4747124EN.pdf

[alfe]

Hi alfe

 

A while back I followed up on your recommendation of a Plextor LB950UE as a good quality option for occasional CD replay.

 

Via the Plextor I really quite like CD replay via HQP or my Bryston BDP and wondered if you felt the Plextor was about as good as it gets or if there is a current external drive out there to better it for ultimate SQ?

 

Thanks again

 

The Plextor is a good product add a good LPS use it as a standalone on flat surface and you are done.

[Norton]

The Plextor is a good product add a good LPS use it as a standalone on flat surface and you are done.

 

Looks like I'm all sorted then!

 

Thanks again

[alfe]

Conclusion:

If all the ripping software are equal the drives are not.

You have a better result with single wavelength drives avoid high aperture ones.

Regulated power is a must have.

Avoid slim drives and slot ones.

Avoid heat, a pause between two CD's when you rip is welcome.

And don't forget the optical drives are not designed for audiophiles

[kumakuma]

Conclusion:

If all the ripping software are equal the drives are not.

You have a better result with single wavelength drives avoid high aperture ones.

Regulated power is a must have.

Avoid slim drives and slot ones.

Avoid heat, a pause between two CD's when you rip is welcome.

And don't forget the optical drives are not designed for audiophiles

 

When you refer to "better result", are you referring to better sound quality when playing a CD, better ripping performance (less errors), or rips that sound better?

[livelistenlearn]

thanks, alfe = educational.

 

Q: any suggestions on OSX software to burn better-sounding discs for in-car use? still using Disco...

 

cheers.

[alfe]

When you refer to "better result", are you referring to better sound quality when playing a CD, better ripping performance (less errors), or rips that sound better?

 

See you coming Tom if we are heading to the bits are bits theory then they better be on time.

PCM audio is two component, bits which give a signal value and step time which give the sample rate, for SQ you can't have one without the other no matter if it's playback or rips.

[alfe]

thanks, alfe = educational.

 

Q: any suggestions on OSX software to burn better-sounding discs for in-car use? still using Disco...

 

cheers.

 

 

Quick answer Nero at 1x

[alfe]

Talking about optical drive lead us to the ripping process of CD audio witch was designed to play continuously and not to be read as individual sectors.

This series of articles called" what the hell is happening to the bits before they get static in an HDD":) may help to understand the ripping process.

 

CD rom sectors are 2352 bytes long divided into 2048 bytes of data plus 304 bytes of synchronisation, header and additional ECC information that are used to control positioning and free errors read. For audio all the 2352 bytes are used for audio data and the only way to address an audio sector is to use the Q sub code information.

Good understanding of the encoding of an audio CD may help: Chip's CD Media Resource Center: CD-DA (Digital Audio) page 9

http://www.computeraudiophile.com/f8-general-forum/encoding-and-decoding-cd-22848/

https://en.wikipedia.org/wiki/Compact_Disc_subcode

 

Timing again:)

[elcorso]

Quick answer Nero at 1x

 

Hi Alfe,

 

Thanks for all you detailed explanations (after & before) !

 

Is Nero at 1x good for CD ripping to hard disc also? (I own a Plextor CD only with some good LPSU).

 

Thanks,

 

Roch

[alfe]

Hi Alfe,

 

Thanks for all you detailed explanations (after & before) !

 

Is Nero at 1x good for CD ripping to hard disc also? (I own a Plextor CD only with some good LPSU).

 

Thanks,

 

Roch

 

Hi Roch,

 

I personally use Nero for ripping my disc, I always run a CRC 32 check before starting ripping and then compare at the end.

The speed is depending of the condition of your disc but if you want to go safe route 1X will do.

 

cheers,

Al

[alfe]

From this document: http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-130.pdf

A section or a sector call it the way you want:) is 98 F3 frames.

F1 frame is 24 bytes audio data at the input of the CIRC encoder.

F2 frame is 32 bytes at the output of the CIRC encoder.

F3 frame is F2 with a control byte input to 8-14 encoder.

For error correction purpose the 24 audio bytes in a frame do not represent consecutive audio samples (p.35) in fact the longest delay between input and output from CIRC encoder is 108 F1 frame times.

The Q sub code determine the sector identity but the audio data are spread over 108 sectors instead of 98 and there is no clear definition of witch audio audio samples belong to the time frame covered by the sector determined by the absolute time of the Q sub code.

I will explain later the interpretation of the drive manufacturer to this degree of freedom.

Before that, next is how to store jitter:)

[alfe]

the run-lengths represented by the pits and lands on the disc, have discrete values, determined by the EFM modulation. The nominal RUN-LENGTH of pits and lands are:3T,4T,5T,...,11T,whereT=1/fEFM-clock =1/4.3218MHz=231nsec.

The actual long-term average length of a pit or land of run-length nT is called the EFFECT LENGTH.
The difference between the momentary length and the long-term average length of a pit or land of run-length nT is called the JITTER.

3T is represented by 001....... 11T by 00000000001.

For example in discs with long playing the linear velocity reaches its lower limit, which means that the pits and lands become shorter (higher density). Therefore the pits and lands representing the higher EFM frequencies (I3) (P 8-13 ECMA doc) are closer to the optical cut-off frequency, resulting in smaller amplitudes in the read-out signal.

Offset drift of the spindle motor may affect the bit size, higher is the jitter more is difficult to the reader to make a difference between 3T and 4T or 4T and 5T.... witch lead to a variation of amplitude of the original signal.

 

 

 

Next little thumb and checksum:)

[alfe]

3 statements:

 

-All form of error checking involves adding something to the digital pattern.

 

http://ccsun.nchu.edu.tw/~imtech/course/ods/Chapter%203%20-%20Error%20Correction.pdf

 

-CRC are based on polynomial arithmetic, for example CRC 32 will detect all errors that span less than 32 contiguous bits within a packet and all 2 bits errors less than 2048 bits apart (wonder why CD rom data is 2048 byte, may be easy computation:))

Undetected errors when data are spread on different block are explained in this document: ftp://ftp.cis.upenn.edu/pub/mbgreen/papers/ton98.pdf

 

-CRC performance is independent of data values it’s only the pattern of error bits that matter http://users.ece.cmu.edu/~koopman/pubs/KoopmanCRCWebinar9May2012.pdf

 

Something to digest before starting (respectfully please I hate being called dumb) this controversial subject

 

Next drive manufacturer interpretation.

[edbk]

Thank you Alfe, very interesting.