Digital vs. Analog Part Two

Are you ready for a fantastic primer of the advantages of digital and analog sound? That's this ongoing column by Josh Gizelt; see the FSD for July 24 for part one. This is part two, in which he runs down the advantages of analog and digital, respectively.

By the way, you know you've bought an old CD when one of the booklet panels has that long explanation of the difference between AAD, ADD, and DDD. -Lukas K.

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ANALOG RECORDING

Analog has several disadvantages.

Tape Hiss and Surface Noise

In the process of recording a tape, the sound of the tape moving across the head, or in cutting a record, the sound of the stylus penetrating the vinyl is added to the sound of the recording itself. Furthermore, the act of playing back the material introduces that incidental sound, once again of the tape moving over the head and of the needle sliding through the groove.

Something of a solution to this problem was pioneered by Dolby Laboratories for tape reproduction, and later optical film sound. A noise reduction system employed at the recording stage boosts certain frequencies that coincide with the sound of the tape hiss. These frequencies are then reduced exactly the same way in playback. The result is that the sound of the tape hiss is significantly reduced. What little tape hiss is left, actually has a positive effect; it seems to add "presence" to a recording. Studios used a constant monitoring program known as Dolby A, while a simpler version was introduced for home use known as Dolby B (when there is a Dolby System symbol on your tape and says nothing else, it is Dolby B). Later, they refined the process and Dolby C came out for home use. Then Dolby introduced the Spectral Recording system for studio and film soundtrack reproduction; a home version of which, Dolby S (which is compatible with Dolby B), exists, but is only on the most expensive tape deck models. There are other excellent noise reduction systems, such as DBX, available, but Dolby is the most well known.

However, whether it is optically or magnetically read, the digital signal is extracted from the source and it is the signal alone that is reproduced, not the reading of it. As a result, the act of playing back a digital recording does not introduce any additional sound.

Generational Loss

When an analog signal is reproduced, each generation it goes through introduces anomalies and information loss, and decreases the signal-to-noise ratio. Anyone who has a copy of a copy of a copy of a tape would know what I'm talking about. By the time you get to the fourth generation, the music is less distinct and the there is tape hiss up the wazoo (of course, none of us have ever copied a commercial tape... RIGHT?).

This becomes a serious issue when an analog recording is mixed. Since every time you make a copy of the sound it is further distanced from the original, a recording that requires many mixdowns is going to display serious problems. Recording musicians will recognize this as being the most formidable problem with "bouncing" tracks on their 4-track tape decks.

In digital reproduction, this is not an issue for the same reason tape hiss and surface noise is not a problem. You are not reading the digital tape itself, only what is encoded on it, so every copy made is identical to the original.

Wear and Longevity

Analog sound reproduction for the most part involves the playback medium touching the surface of the reading device. As a result, every time you play a tape or spin a record, you are wearing it down and accordingly decreasing sound quality.

The exception to this is when the sound is read optically, by shining a light through a pattern, which is how analog film sound works (humorously demonstrated in a sequence in Fantasia). However, while the optical soundtrack on a film can be read without touching the film, the act of running the film itself often causes wear on the material, and ambient light introduces additional sound. But, there is another form of optically read analog sound that will be discussed later.

The CD and mini-disc solve this problem by being optically read. The DAT and DCC do not. For the first part of the tape's life, it will play exactly the same each time. As the tape ages, however, dropouts and artifacts (loss of signal or additions to signal, such as a metronome sound) will be introduced as the tape becomes worn.

DIGITAL RECORDING

The pitfalls of digital recording (no pun intended) are as follows:

Losing Material

Since the original wave is being cut up, some sounds will "fall through the cracks," so to speak. How much this happens depends on the bit-rate being used. Most digital recordings and compact discs utilize a 16-bit system. More recently, we have seen 20 bit recordings, and now 24 bit recordings are showing up. These show a marked sonic improvement, as additional sets of bits increase the amount of information exponentially.

The Essence of the Material Is Altered

Because the original sound wave is being approximated, the sound has essentially been diced, and when it is put back together, it is stepped. This means that the curve of the wave is no longer smooth, but instead somewhat jagged. The overall sound suffers from this. The extent of the problem is lessened (but not eliminated) by increasing the bit rate. The most demonstrative evidence is the metallic sound in the strings in many digital recordings.

There Is Rarely "Presence"

When making an acoustic recording, in addition to the instruments, the sound of the room, is recorded. Reverberations and echoes are natural, but so is a concept that filmmakers refer to as "room tone."

No room is completely silent unless it is airtight, soundproof and there is no human presence. When extraneous sound is absent, anything recorded sounds like it is disembodied in space. Because the recording process works to eliminate external noise, factors such as directional microphones will reduce and remove "room tone."

In a digital recording, in addition to the loss of parts of the overall sound, there is no sound of tape hiss or stylus noise. While these are artificially produced, the effect of these (usually white noise) additions often make the recording sound like it has a greater presence than the relatively sterile digital sound.

So, as you can see, there are a few major strikes against digital recording.

TO BE CONCLUDED...

Tomorrow: Be here for "This News Friday" with Lukas.