CD Player FAQ

What should I listen for when evaluating a turntable or CD player?

For tape decks and turntables, beware first of speed variations (wow and flutter). A good check for this is Richard Strauss' "Also Sprach Zarathustra" (aka: The Theme From 2001), which has a long, low, sustained organ note that comes in well before the main theme starts, and is held through the first movement. Concentrate on that. Make sure it doesn't wobble or warble. There's also a good bit at the beginning of Pink Floyd's "The Wall", but it doesn't go on as long, so you've got less time to think about it. Tape decks are prone to losing high-frequency notes, so pick something you like which has lots of treble, and make sure it is clear.

The sound of a turntable is largely bound up in the kind of cartridge mounted on it. Make sure to listen to a table with a cartridge similar to what you're buying, and not one in a different price bracket. If possible, audition the turntable with the same arm and cartridge, so that you will experience potential cartridge/arm interactions, too. Most cartridges work better with one arm than another. Treat the tonearm/cartridge pair as a system, rather than independent parts.

For CD players, try some piano music. See if the high notes sound tinny. Also, try something which has some soft parts, not the same as turning the volume down. Distortion for CD players (as for other devices) is measured at a high output, but in fact in CD players (unlike others) it's likely to be worse in soft passages of music. Most classical recordings contain a suitable soft passage. Most rock music won't.

Distortion in CD players, if you want to call it that, is a function of the granulation noise, or time-delay pre-echo that can come out of the filtering. To listen for this, use material that is rich in high-order harmonics, such as brass music. Unfortunately, you can't reliably predict how a CD player will sound by looking at specifications, features, or the technology it uses. If you want to know how a player will sound, you MUST listen to it.

Are some discs better than others?

Some recordings are better than others. Some artists are better than others. Some recording engineers are better than others. Some microphones are better than others. Some music is better than others.

Ignoring that, there is some difference between discs. Some of the very earliest discs were badly made and deteriorated with time. The technical problems that caused those problems have been solved.

Some "gold" discs are available which are advertised to have better life and quality than common "aluminum" discs. These sell for an extra US $15 or more per disc over the cost of the same music on a common disc. Studies have shown that there is an advantage to glass-encased, gold platters for archiving computer data that is not error tolerant and will need to be stored for many tens of years. I have yet to see a similar comparison which justified any extra effort for storing audio recordings for 50 years. Part of the reason for this is that audio recordings contain error correction codes, allowing a CD player to perfectly reconstruct minor flaws. Another reason is that CD players can effectively reconstruct badly damaged audio data, even if some data is completely missing.

Some discs seem to have pinholes in the aluminum, which are visible when the disc is held up to a strong light. However, these discs play fine and last very well, so the effect of these pinholes is probably nil. Some have performed studies counting errors on various discs with various players. They found that, in general, the error count was consistent from one player to another. Also, in general, most discs have a low, consistent error rate which is perfectly correctable using the redundant data stored on the disc. This study did find that one group of discs had a higher error rate than all of the rest. This group was the promotional discs, also called "music samplers" given away by music companies to introduce you to their family of artists and performers. Despite these higher error counts, these discs still played fine.

If there is no abusive handling involved, I have rarely heard of a disc that degraded with time. Of the few that have existed, they tended to be from one of the bad batches mentioned earlier.

There is no doubt that some discs are mastered better than others. Some are badly mixed. Some are so badly recorded that there is noticeable clipping. Some are made from damaged master recordings. CD technology is no guarantee of good music or of a good recording.

Are CDs better than LPs?

Some excellent recordings are mastered digitally, and sound great on LPs. This suggests that there is nothing inherently bad about digital.

Some find that LPs sound better than CDs. Advocates of LPs claim that the digital to analog (D/A) converter in home CD players isn't up to the quality of the information on the disc. They also claim that the analog electronics in a home CD player can be poor.

Some believe that CDs do not sound like LPs because the CD does not have the frequency response errors, the distortion, or the stereo separation problems of LPs.

In general, though, there are good and bad CD players, just as there are good and bad turntables, cartridges, and tone arms. Any ultimate comparison would require ultimate equipment, which is unaffordable. In moderately priced systems, there will be some signal damage from the turntable system and some signal damage from the CD player.

LP lovers often learn the nuances of cartridge selection, record care, and even turntable and tonearm adjustment. They have found that the turntable will sound different if the arm height is adjusted, if the cartridge angles are changed, and if the tonearm wire is moved. CDs do not offer as many avenues for the home experimenter.

However, Audio Amateur Magazine has published modification projects for CD players; particularly for Magnavox 560 and similar European players. Audio Magazine has also published such articles.

What about CD green pens?

In a nutshell, save your money.

A CD player "reads" information on the disc with a laser light beam. Some believe that if you put a green stripe on the very perimeter of the disc, then the light beam will not reflect around inside the disc and will more clearly pick up the data.

Scientific studies of the data coming off of the disc have failed to show any difference between a virgin disc and a green painted disc. I have not heard of double blind listening comparisons that have proved that there are people who can hear the difference, although many have performed uncontrolled tests with positive results.

What about CD stabilizer rings?

In a nutshell, save your money.

The data coming off of the disc is a serial string of ones and zeros. If this bit stream has jitter, then it may reach the D/A converter out of sync. If this happens, then the actual analog signal recreated will have jitter, and won't be perfectly true. The vendors of stabilizer rings say that using these rings will reduce jitter and make a more perfect signal. Vendors also claim that the rings can increase the mass of a disc, making it spin more smoothly, and reducing transient load on the power supply from the motor.

Some players will not play discs that have stabilizer rings on them. The clamp can't handle the thickness. Other players play ringed discs, but do not play them well, because the disc motor was not built for the added load.

With those exceptions, scientific studies of the data coming off of the disc have failed to show any improvement going from a virgin to a ringed disc. I have not heard of double blind comparisons that prove that people hear the difference, either.

Are 1-bit CD players better than multi-bit players?

In a nutshell, they are virtually the same.

There are some excellent sounding 1-bit players and some excellent sounding multi-bit players. Some feel that the 1-bit technology has more future because it can be improved with the rapidly improving digital technology, while the multi-bit players improve with slowly improving analog technology. Multi-bit also has its advocates.

All of the various D/A converters try to do the same thing, and try to achieve the exact same ideal performance. How well they succeed is more a function of their skill and the quality of the parts that they buy than the technique that they use. In other words, the architecture of a D/A converter is less important than the quality of its implementation.

Are three lasers better than one in CD players?

Some players have one beam, some three. All use one laser diode to generate the beam. Three-beam is just a different method for doing track alignment. Neither is better than the other.

There are good 1-beam players and good 3-beam players. Manufacturers want advertising claims and "More Beams Is Better" sounded good to some marketing people. Trust your ears.

What are the differences between multibit and Bitstream/MASH Analogue to Digital converters (16-bit vs 1-bit CD players)?

Audio data is stored on CD as 16-bit words. It is the job of the digital to analogue converter (DAC) to convert these numbers to a varying voltage. Many DAC chips do this by storing electric charge in capacitors (like water in buckets) and selectively emptying these buckets to the analogue ouput, thereby adding their contents. Others sum the outputs of current or voltage sources, but the operating principles are otherwise similar.

A multi-bit converter has sixteen buckets corresponding to the sixteen bits of the input word, and sized 1, 2, 4, 8 ... 32768 charge units. Each word (ie sample) decoded from the disc is passed directly to the DAC, and those buckets corresponding to 1's in the input word are emptied to the output.

To perform well the bucket sizes have to be accurate to within +/- half a charge unit; for the larger buckets this represents a tolerance tighter than 0.01%, which is difficult. Furthermore the image spectrum from 24kHz to 64kHz must be filtered out, requiring a complicated, expensive filter.

Alternatively, by using some digital signal processing, the stream of 16-bit words at 44.1kHz can be transformed to a stream of shorter words at a higher rate. The two data streams represent the same signal in the audio band, but the new data stream has a lot of extra noise in it resulting from the word length reduction. This extra noise is made to appear mostly above 20kHz through the use of noise-shaping, and the oversampling ensures that the first image spectrum occurs at a much higher frequency than in the multi-bit case.

This new data stream is now converted to an analogue voltage by a DAC of short word length; subsequently, most of the noise above 20kHz can be filtered out by a simple analogue filter without affecting the audio signal.

Typical configurations use 1-bit words at 11.3MHz (256 times over-sampled), and 4-bit words at 2.8MHz (64 times oversampled). The former requires one bucket of arbitrary size (very simple); it is the basis of the Philips Bitstream range of converters. The latter requires four buckets of sizes 1, 2, 4 and 8 charge units, but the tolerance on these is relaxed to about 5%.

MASH and other PWM systems are similar to Bitstream, but they vary the pulse width at the ouput of the digital signal processor. This can be likened to using a single bucket but with the provision to part fill it. For example, MASH allows the bucket to be filled to eleven different depths (this is where they get 3.5 bits from, as 2^(3.5) is approximately eleven).

Lastly it is important to note that these are all simply different ways of performing the same function. It is easy to make a lousy CD player based around any of these technologies; it is rather more difficult to make an excellent one, regardless of the DAC technology employed. Each of the conversion methods has its advantages and disadvantages, and as ever it is the job of the engineer to balance a multitude of parameters to design a product that represents value for money to the consumer.

All sampling techniques (so also D/A techniques) require an analog reconstruction filter following the converter. This filter inherently adds phase shift, frequency response ripple and high frequency roll-off, depending on the characteristic of the reconstruction filter (which depends on the position of its poles and zeros).

An oversampling data converter generates a higher output sampling rate than a simpler converter, so you can use a more simple reconstruction filter, which is cheaper and more stable in time and temperature and produces less noise. Also, modern oversampling systems include digital filters which compensate the response of the analog filter in the passband, so you can achieve systems with an overall performance of 20 Hz...18 kHz +/-0.05 dB. Also deemphasis is mostly done in the digital domain.

So the "sound" of a CD player is more than just the number of bits. It's the quality of the converter, the filter requirements imposed by that converter, the quality of the filter, and of course, the quality of the following analog components. Power supply quality and clock jitter also influence the sound.

Can I add digital output to a non-digital-out CD player?

Some Magnavox CD players using the Philips chip set can be modified. Look for a SAA7220 IC. If it has one, then it can be modified. If you have experience modifying electronic equipment, follow this procedure:

Take pin 14 of the SAA7220 IC and remove whatever terminating resistor is on it. Connect it through a 560 ohm resistor to the input of a wide band pulse transformer. Tie the other end of the primary of the transformer to ground. Pulse Engineering PE65612, Schott Corp 6712540, and Scientific Conversions SC916-01 all will work. Bypass the primary through a 620 ohm resistor. Connect the output of the transformer to an RCA jack. Do not ground either side of the RCA jack. This output is now S/PDIF compatible. (Thanks for the tip to Positive Feedback)

What can I get in the way of a CD test disc?

Each test disc offers something different. Some discs contain useless filler which advertises a product or shows a unique capability, but really doesn't help you test or improve your system.

Many use the Hi-Fi News & Record Review test discs. So far, these have received only positive comments.

Chesky produces 2 test discs. The first, "Chesky Jazz Sampler Volume I" contains some excellent imaging test signals (called LEDR), some well-recorded acoustic jazz, and other test signals. The second, "Chesky Jazz Sampler Volume II" has similar music & different tests.