Point of View
We asked Australian
to answer the following question...
What is the difference between
digital and analogue recording?
with help from Robin Gist
I'd like a dollar for every time I've been asked this question and a
dollar for every different point of view on the subject. It seems like a
simple question and if there was a simple answer it might be that
analogue IS the waveform and that digital is a SAMPLE (or a numerical
representation) of the waveform. This, however, is not telling us much
about the audible differences and only fuels the hot debate over what's
the best format to record to. One thing is very clear and that is that
people are pretty much either in one camp or the other. I'm not going to
try to say one platform is better than the other here, just that they
have known, measurable differences. So let's explore what the
differences are in a bit more detail.
Analogue recording setups generally include microphones that "hear"
or detect the air pressure changes from a sound source and then generate
a small, changing voltage over time at the mic's output socket (a
waveform). This waveform is fed into a mic pre-amplifier and that same
waveform is then amplified up to "line level" (the sort of level that
most audio equipment works with) and this is what is fed into an
analogue tape recorder, sometimes via a compressor and/or equaliser (EQ)
that modifies the waveform somewhat. The resulting waveform that is
recorded is still the original air pressure changes represented as a
changing voltage over time from the mic with some slight changes due to
the compressor or EQ.
To record this waveform, the recording device ultimately routes the
electrically changing signal to a small coil of wire built into a
housing, which is otherwise known as a record head. Thanks to the almost
magical phenomenon of electromagnetism, the changing electrical signal
in the coil produces a varying magnetic field or flux around the head. A
very sensitive magnetic material - usually ferric oxide (rust) bonded to
a plastic backing medium (recording tape), is passed over the head at a
constant speed. As the tape passes over the head and through the varying
magnetic field (a direct analogue of the original waveform) a
representation of the field is stored in the tapes magnetised particles.
Analogue tape recorders have separate heads that are used in the
playback function. When the machine is made to play back the recording,
the magnetic tape is moved over the replay head. As the magnetic pattern
in the tapes particles moves past the head, a small voltage is created
in the coil in the playback head that is then amplified back up to line
level (electromagnetism works both ways!). This line level
waveform/signal is then sent to a more powerful amplifier that drives
your room speakers connected to it and the result is a waveform moving
the speaker coil in a way that resembles the original waveform that the
mic detected in the first place. It might be said that the original
pressure wave was sent through various electrical circuits, converted to
a varying magnetic field and stored magnetically, then replayed and sent
to a device that creates air pressure changes (a speaker) and is the
same waveform all the way.
With a digital recording setup some things are still analogue but not
all. You still have the same analogue microphone to "hear" the air
pressure changes from the sound source that generates a voltage at the
mic's output socket. This signal still needs to be amplified up to line
level with an analogue mic pre-amp and then this signal is then sent to
the all-important analogue to digital converter (A to D).
An A to D converter can either be in a computer, inside a stand-alone
digital audio recorder or can be just a converter on its own. Think of
an A to D converter as like a cold meat slicer that chops the variable
analogue signal into slices or "snapshots" of the analogue signal's
voltage level at that instant in time. The number of slices you take per
second is called the samplerate, and the higher the samplerate, the
better the data resolution. A digital clock drives the A to D chip to
sample at typically 44,100, 48,000, 96,000 or 192,000 times a second
that results in a continuous data stream of 16, 24 or 32 bit binary
"words" that is then stored on a hard drive (again using
electromagnetism) or alternative digital storage media (memory sticks
To convert our data stream back to an analogue signal, we need a
digital to analogue converter (D to A). The D to A chip takes the data
stream and generates a voltage on it's output. As the binary numbers
pour in, the voltage on the output keeps changing and presto an analogue
waveform! It is, in theory, just like the original analogue waveform
that was sampled in the first place. This waveform/voltage/signal is
sent to an amplifier usually in the converter first to get it up to line
level. Then this goes to the main power amplifier that drives the room
speakers. The speakers modulate the air pressure in the room and there
is the sound again: produced by sampling a waveform, representing is as
a binary number and storing the resulting data stream, NOT the actual
wave. This is for me, the real difference between analogue and digital!
The big question is: "Which is better - analogue or digital?" Well,
this is where it gets tricky. "Beauty is in the ear of the beholder".
There is much to be said about recording actual wave energy and then
using this stored energy to re-modulate the air to make sound again.
Obviously though, the energy from the sound of a real drum for example
live in a room, is more than gets stored when you try to record it,
digital or analogue. To reproduce a sound with a speaker and amplifier,
with that much energy, requires a lot of power. This is why there are
all these amplifiers along the way. Only a small amount of the energy
from an analogue sound in the air is stored to the recorder.
In the analogue domain there are lots of variables. The type of oxide
on the recording tape for example affects the tone of the recording
slightly due to the chemical coating being slightly more or less
sensitive to the magnetic field and therefore slightly changing the
waveform. Any variation in the speed of the tape player, slight slowing
down or speeding up for example, also changes the waveform by
effectively stretching or shrinking it over time (wow and flutter). The
overall volume range that you generally have to work in is much less
than in the real world. Analogue tape also comes with a relatively high
noise floor due to friction noise from recording tape passing over the
heads. This has been very cleverly hidden with modern noise reduction
systems but they require very fancy encoding modifications to the
waveform during the recording stage and are then changed back in the
playback stage (decoding). This returns the waveform to how it was but
can contain some slight artifacts. Often, these are thought of as tonal
Digital recording is advancing all the time. Some of the first
products available used low sample rates and low-grade electronic
components and didn't do a very good job of trying to describe or
capture complex waveforms. To record music, sample rates need to be
high. This is very important. Some instruments in the orchestra have
incredibly high frequency content. Cymbals have frequencies out to 35 or
40kHz. This is way above what humans can hear so what's the problem you
ask? The problem is that some frequencies modulate against other
frequencies. For example, if two violin notes play together but are ever
so slightly different in pitch, they will chorus or phase against each
other. This inter-modulation happens right across the audio spectrum and
is one of the things that give our brains acoustic detail. Even if we
can't hear some sounds because they are out of our range, it doesn't
mean they aren't having an effect on the frequencies we can hear. Many
very expensive and high-grade pieces of analogue audio equipment have
frequency responses far beyond human hearing and are easily able to
record signals at extended high frequencies.
The standard "red book" CD digital audio format of 16bit/44.1kHz only
lets you record or reproduce to 22kHz (Google Nyquist theory if you want
to know more about this). As processing speeds and the capacity of large
storage devices increase, we have seen and will continue to see better
digital sound quality and cheaper storage. I like to record audio
frequencies out to 40kHz. This means I set the sample rate of my A to D
to 88.2kHz and 24-bit depth. The bit depth is what determines the
available dynamic range of an A to D and 24 bits gives me about 118dB of
dynamic range (far beyond any analogue recorder that I know of).
Recording at 88.2kHz sample rate also allows for an easy "down
conversion" to 44.1kHz for red book audio CDs.
Another difference between analogue and digital recording is that the
production methods can be a bit different particularly with the ability
to "cut and paste" in the digital domain. Often, a digital studio is
smaller and many are home or project studios. This usually means that
the instruments are recorded separately. If you have an analogue studio,
you generally have physically bigger equipment and therefore a larger
studio to work in. Recording drums, bass, guitars and keyboards at the
same time you're getting vocals, gives the music a more integrated
sound. The musicians work better with each other if they are in the same
room together. Often this more cohesive recording sounds more like an
old style analogue recording.
I think that these days both analogue and digital recording systems
are of a very high standard and quality. Yes, some systems sound
different to others but are still generally very good. In my opinion, it
is down to the sounds that you record. A good microphone in the right
spot on a quality instrument with a good player can sound very good,
digital or analogue. The reproduction quality is only as good as the
playback equipment to some degree anyway (think about MP3's!) and if
that equipment is not set up in the right spot in the room or with wrong
settings on the EQ, this would have a far greater effect on the quality
of the sound anyway.
To summarise, there are many differences between digital and analogue
and the argument on what's better will go on for a while yet. Digital
will get better with time but analogue will also, to some extent. My
experience is that, seemingly redundant technologies still hang around
long after their supposed "use by" date and at least for the moment, you
– the engineer/producer/musician have a choice between the two formats.
© 2010 Steve Newton
|Scroll down the page to post your comments.
Thanks, Steve, for a very interesting and for me, very informative article. I must confess that I don't really hear the difference in analogue or digital recordings as far as sound quality goes. The main ingredient in either form of recording, will always be the performance of the players involved.
Posted by Ian Beddows on Thursday 1 July 2010
Hi Steve. Thanks for your interesting article. I read a long interview that Audio Technology magazine did with Rupert Neve. He discussed the lie sold to the public about only being able to hear up to 20,000Hz in order to push the CD media through as a replacement for Vinyl records and Cassettes. A similar story to VHS and Beta video tapes. His conclusion was that we don't necessarily hear those higher frequencies as sounds, but rather as feelings, and when you remove frequencies above 22,000 Hz (or actually 22,000 Hz), and also certain frequencies below that, you introduce anxiety into the sound. If you remove the resolving harmonics of a sound and force the listener to make up the missing frequencies, you cause the sound to become fatiguing.
He made references to the difference between our tolerance for old records versus CDs, particularly our ability to sit through a whole album without becoming agitated. My experience matches his comments (as do Andy's below). I feel as though I can emulate a similar smoothness and coherence to tape when recording at 48/192 24 bit, but it all comes back to 44.1/16 when you press it to CD, and dithering adds nasty artefacts too. So where does that leave us? Make SACDs! That's what I think is the closest and best sounding medium by a long shot, or DVDAs. Then go and buy everyone a player so they can hear your wonderful album....
Posted by Bill Risby on Wednesday 30 June 2010
Congratulations Steve for an excellent and very interesting article mate!
From my point of view, I've never been truly happy with anything that I've recorded via digital means. I remember the crispness and warmth of the recordings I did at the ABC via Cleon Dennis and Julian Lee in the 1980's. It may be just happy memories of great times recording with Sydney's elite at that time, but I remember the big analogue machine tapes that were so preciously marked and stored at the end of each session. The sound always seemed to just jump out of the speakers and "grab you". I have pro tools with a nice Mac quad G5 set up here at home but I've never been able to get the same warmth and "charm" of the analogue recordings that I've done. I still collect the great jazz records I had as a kid on Ebay and in the various second hand bookshops around the country as I find them. Even though I have the CD recordings of these, they never sound as good to me as the records. Even my 1922 gramophone has a certain indefinable grittiness about it and all of the instruments on the discs are just so clear and bright! I am definitely a fan of digital music's storage and ease of use/manipulation but for sound, give me analogue any day. Once again, thanks for a GREAT article. Most enjoyable!
Posted by Andy Firth on Wednesday 30 June 2010
|Post a Comment