According to the high-end Hi-Fi connoisseurs' bibles, for a perfect reproduction of the sound, in addition to "special cables", etc., you must have a specialized stage of CD which reads the optical disc and outputs the digital signal.

Assuming that the disc is read flawlessly, the signals should be identical.

If there are errors, the error correction algorithm is still the same as long the number of errors is within the capacity of the error correction algorithm. Beyond this, any "fix" is simply noise.

As I know, and I may be wrong, a very small number of companies effectively produce the CD players' mechanical and optical units, and therefore at least these parts are common to both the cheap and the costly units.

So what makes the difference?

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    \$\begingroup\$ There are reasons that "audiophiles" are sometimes referred to in engineering circles as "audiophools". This is one of them. \$\endgroup\$
    – brhans
    Oct 4, 2022 at 8:35
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    \$\begingroup\$ Bandi, there's two really wholly different things. One is "just" the part that reads digital data off the disk. The other is "just" the part that converts digital data to an audio signal. It's unclear which "part" you mean, or do you mean both? \$\endgroup\$
    – Fattie
    Oct 4, 2022 at 17:37
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    \$\begingroup\$ Techmoan has a pair of videos on youtube that look at "CD demagnetizers" and "Shaving the edges of CDs" - both for "increased fidelity". They're hilarious. youtube.com/watch?v=mH4v8b1tGSQ & youtube.com/watch?v=f-QxLAxwxkM \$\endgroup\$
    – enhzflep
    Oct 5, 2022 at 19:25
  • \$\begingroup\$ Be sure to only eat foods that are organic, non-GMO, keto, paleo, range-free, cage-free, gluten-free, natural, vegan, unprocessed, antibiotic-free, hormone-free, have no added sugar, and are triple the cost. \$\endgroup\$ Oct 6, 2022 at 20:19

5 Answers 5


Assuming you read this answer...

Under normal conditions, the data encoded in the digital output is what's on the disc. You can easily verify this by recording it using a soundcard with SPDIF input and comparing digitally with a rip of the CD using PC software. I tried this with various CD players and CDs with different amount of scratches and could confirm that, indeed, it works.

The exception is CD players with digital volume control that also operate on the digital output. In this case, sample data will be multiplied by a constant which is your volume setting. This can be done properly: ideally 16-bit data is multiplied by volume and output as 24 bit digital, or it could be dithered to 16 bits which will lower SNR at low volume settings. However, some players, notably CD723 and most likely all other players using the same decoder chip, have a buggy volume control which multiplies then truncates without dithering. In addition it cannot be set to a multiplier of 1, so at full volume it multiplies by something like 0.99 and truncates. This causes an increase in quantization noise which manifests as a loss of detail. It's baked in the digital data, so it's not possible to get rid of it. The only solution is to avoid such players... or rather was, because CD is obsolete.

Besides that, the bits that come out are what's on the disc, period.

The same problems can occur with soundcards, it takes quite a bit of effort to ensure the bits coming out are actually the bits being played. Sometimes the drivers or OS will sneak in some resampling, volume control, or other effects. It is best to check with a loopback recording using the SPDIF input.

The other problem is jitter, that's either "the plague" or "woo-woo" depending on who you ask. In my experience, it's quite audible, although various DACs have wildly differing sensitivity. A proper DAC should have zero sensitivity to it, but it is quite difficult to achieve. When a DAC has high sensitivity to jitter, it will sound different with different CD players or digital cables, then audiophiles call it "transparent" while it's actually a problem with the DAC. However, one might want to still make the DAC's job easier by using a low jitter source, or at least avoid the most common sources of it.

Here's a scope shot of CD723 digital output from my archives, it has a very slow rise time and a large amount of jitter, about 3ns, visible on a scope.

enter image description here

This is measured by triggering on an edge, then adjusting the timebase delay to display the next edge, so the scope displays the time delay between two edges, which should be constant. In this case it is not constant, so the trace is smeared horizontally.

Jitter on a SPDIF output can be measured with expensive equipment like Audio Precision.

The homegrown way to do it is to use a SPDIF receiver which does not attenuate jitter (like CS8412), input a SPDIF signal, and acquire the receiver's WCLK output with a soundcard. If the digital audio runs at 44.1k, wordclock will be at 44.1k too, so the soundcard should be set to 192k sampling rate to acquire it. Then, you can do a FFT, display it, and look at the phase noise skirts, or do any other kind of processing you like. It is quite instructive.

You can also look at the recovered bitclock with a spectrum analyzer, or mix it with a known stable frequency and look at the resulting frequency difference signal with a soundcard. There are plenty of cheap ways to do it, so if you want to "tweak" a digital source it would be a good idea to setup a jitter measurement rig to at least have an idea of what you're doing.

From my experience, jitter in a CDP digital output comes from:

  • Cheap crystal oscillators

That's the #1 cause, because an oscillator is an analog circuit working with rather low level signals, so it is quite sensitive to noise. It is usually implemented as a crystal slapped on the big decoder VLSI which is the worst case, because these chips are very noisy. Add a noisy power supply and double sided layout with no ground plane, and you can be sure the performance will be way below what you would get with any €1 canned oscillator powered by a dedicated 30c LDO. In addition, crystals are quite sensitive to vibration. The oscillator is usually the weak spot because power supply noise causes frequency variation which is integrated into phase noise, whereas downstream sources only add phase noise, which is not integrated.

  • Cheap power supplies

The optical pickup coil actuators must follow the track on the disc, so they will draw a current from the supply which depends on the shape of the track, how much the disc vibrates, etc. It's basically a big microphone. Then there will be a voltage ripple on the supply that mirrors any vibration in the device. You can probe the supply rails and tap the player with a finger to see the effect, for example. If If the power supply is too cheap or badly designed, and this supply rail is shared with sensitive components, then you get a problem, and audiophiles will discover it sounds better with those fashionable spiky feet that cost more than the player, or put a brick on it, or mill the edges of the CD to make it more round and centered around the hole so it vibrates less, etc.

  • Bad design

The digital audio output usually comes out of a big noisy VLSI, so the correct way to do it would be to use a fast high slew rate flop to re-align the edges with the clean clock from the canned oscillator just before the output. It doesn't cost much. Also the cable should be driven with proper 75R termination taking the driver impedance into account, no coupling cap otherwise you get intersymbol interference, proper shield termination to enclosure and other usual practices when dealing with highspeed signals on a coax. This never happens, usually you get a 75R resistor, a 10nF cap, and a RCA that isn't even grounded to the metal box.

  • Bad layout

Before 4 layer got cheap, the usual way was a 2 layer PCB without ground plane, and the digital output signal would usually pass through ribbon cables with only one ground wire shared with other highspeed signals. This also adds plenty of noise.

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    \$\begingroup\$ Couple of points. Sound cards generally use their own clocks so recording SPDIF is not bit accurate but resampled (an exception is expensive pro studio equipment maybe). And the classic CS8412 does attenuate jitter with a PLL, it reads right on the first page, and it attenuates it strongly. \$\endgroup\$
    – Justme
    Oct 4, 2022 at 9:11
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    \$\begingroup\$ @Justme A soundcard should sync to incoming SPDIF if it has an input, even the motherboards with spdif inputs do this... some will resample... user should read the manual to make sure :D CS8412 has no jitter attenuation below the PLL bandwidth which is quite high. WM880x has superb jitter attenuation. \$\endgroup\$
    – bobflux
    Oct 4, 2022 at 9:48
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    \$\begingroup\$ Before onboard sound cards on motherboards were even a thing, most prosumer cards ran internally at 48kHz only so no chance getting bit accurate 44.1 kHz from CD. CS8412 begins attenuation from 25kHz and attenuates 50dB at 1 MHz, having 200ps. WM880x is better, 50ps, goes down to 100 Hz. \$\endgroup\$
    – Justme
    Oct 4, 2022 at 10:05
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    \$\begingroup\$ Ah yes that was the famous Soundblaster Live with its always-on resampler! \$\endgroup\$
    – bobflux
    Oct 4, 2022 at 10:27
  • \$\begingroup\$ @PeterCordes Thanks, fixed \$\endgroup\$
    – bobflux
    Oct 6, 2022 at 11:33

In simple terms, it's the jitter and the matching of the digital signal to the DAC input.

When I was at University back in the 90s, I was friends with someone who was both rich and keen on HiFi. He had a whole range of CD players, CD transports, separate DACs, etc. My friends and I had a lot of fun trying to work out why the players, transports and DACs sounded different. What seemed "impossible" was that different CD players, transports, and even the digital cable connecting to the DACs did make a difference.

Most DACs don't have good jitter reduction circuitry, as it can get expensive, so the jitter on the digital input becomes noise + distortion on the audio output.

Similar, if the clock extraction circuitry in the DAC is sensitive to the shape of the digital signal, as that affects how well it can lock its internal clock and the corresponding jitter.

This is why the cable makes a difference - it has a characteristic impedance which affects the shape of the digital signal, which in turn affects the quality of the clock in the DAC.

We made a custom circuit based on a CS8412 + VCXO and re-clocking buffer + CS8406. The VXCO was similar to https://docs.rs-online.com/9015/0900766b80d442b6.pdf (I can't remember the exact part). This reclocked the digital signal and removed almost all the jitter.

With this circuit in between the CD transport and the DAC, it was impossible to tell the CD transports apart. Similarly, you could swap the digital cables between the CD transport and the custom circuit, and that made no difference either.

It also improved the audio quality of the cheaper DACs, as much of the issue was in the digital input.

  • \$\begingroup\$ Fantastic answers here. I should probably ask a separate question, but ... under discussion is digital info coming from "a CD transport". the thing is, these days you can just move the exact unchanged digital data to "the ram in a computer", and throw away the transport and the CD. Then, i guess, the exact same data can be piped? to the DAC. In that case, RogerLucas, is jitter then nonexistent, an irrelevant paradigm? Or do I misunderstand? Thus, for actual Hi Fi Nuts of today as we speak, do they never ever use actual CD transports? they just first pop the data on to ram? TY! \$\endgroup\$
    – Fattie
    Oct 4, 2022 at 17:45
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    \$\begingroup\$ Hi @fattie, the jitter is always present. Loads of PCs have a digital audio output (may be coax or optic) and you are completely correct in that they can replay the audio CD from hard disk or whatever and still be bit exact. The jitter is a factor of the quality of the clocks in the PC audio I/O... but there will always be some. The above circuit would work exactly the same sitting between a PC and the DAC, and improve the jitter in just the same way. \$\endgroup\$ Oct 4, 2022 at 19:48

The digital signal is digital. There is no dfference in the data itself.

If there is any difference, it might be in aspects that are not really relevant at this day and age any more, even if they were relevant when first CD players were made.

More expensive trasports may have tighter tolerance cloks. If they use a ceramic resonator, the sampling rate could have 1% tolerance. If they use a crystal, the tolerance could be below 100 PPM.

The clock might have jitter, which means the edges of two clocks may have a varying period even if the average clock perios is perfect. This used to be a problem with digital interfaces and DACs, if the sampling period is not constant. These days DACs and SPDIF receivers have a PLL in them anyway so they can accept jittery data stream and still work with stable clocks.

Of course more expensive transports can be made otherwise from better electronic and mechanical parts, and so they might tolerate wear and degradation of the parts better. It's not unheard of that one CD player tolerates more scratches than another player.

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    \$\begingroup\$ The second paragraph is key, it's not that high end audio equipment doesn't produce differences, it's that human senses are terrible enough that cheap equipment produces results that are statistically indistinguishable from things 100x the price. See also expensive wines, violins etc etc. \$\endgroup\$
    – eps
    Oct 4, 2022 at 22:20

The ‘quality’ of a CD player has some influence on the digital output. But note that this doesn’t equate to price. An inexpensive player with a good design can equal an expensive player, or even outperform it if the costlier unit has a bad (or misguided) design.

There are three issues at stake when it comes to recovering and playing back the digital stream.

Issue 1: SPDIF phase noise. Your DAC playback system uses the digital (SPDIF) stream to recover its local DAC clock, (unless it resamples, which introduces its own distortion.)

If the player has a lot of phase noise present on SPDIF, and the DAC receiver fails to clean it up, this phase noise may influence playback quality, depending on the DAC’s sensitivity to it. Ultimately this is more of a DAC issue than a transport one.

Issue 2: Transport raw error rate. A good player will have better servo performance and thus by definition have fewer errors to begin with. That said, the raw error rate for CD is pretty abysmal to start with: 1 in 10,000 or so. Which leads to…

Issue 3: Error correction and concealment. If the good-performing servo is also backed up by robust error correction and concealment, then the playback data will be truer to the program material.

In other words, a good player with a robust servo and error correct/conceal will be able to ‘play through the rough’ better than a bad one. Really sophisticated ones may even implement retries (a common feature in CD-ROM) to further reduce the number of errors that sneak through and pollute the data.

Both phase noise and non-corrected or poorly-concealed errors can be objectively measured using test equipment.

Subjectively, playing a clean, flat and scratch-free disc to a DAC that does clean clock recovery will have no human-discernable audible difference on a cheap but well-designed player versus an expensive one.


More expensive CD players have normally better power supplies, with separate transformer windings for transport, digital and analog circuit sections. On the other hand cheap CD players are cutting corners so the actuator and digital noise leaks on the audio path. I have a cheap portable CD player that has that problem.

Another problem with cheap players, that goes in the category "read the disk flawlessly" is that more expensive players are normally built better mechanically and make the laser tracking better and less sensitive to external vibrations and shocks, even if the basic mechanism is the same.


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