If I'm converting something from analogue to digital, I digitise it.
What am I doing if I convert from digital to analogue?
You reconstruct, as in a reconstruction filter found in digital-to-analog converters:
a reconstruction filter is used to construct a smooth analog signal from a digital input
This is an interesting question.
The distinction between the two is characterised by the distinction between the discrete and the continuous. To take something discrete and make it continuous, one interpolates.
Mathematics. to insert, estimate, or find an intermediate term in (a sequence).
I know of no such specific antonym to digitizing; however, by digitizing an analog signal, we are actually encoding it. And, so when we convert the digital signal back to analog for consumption, we are actually decoding it.
1.2 Convert (audio or video signals) into a different or usable form, for example to analogue from digital in sound reproduction
Perhaps the simplest solution here is to use the prefix "un-":
Some might argue that a hyphenated word is not a single word, and some might suggest that the above is not a "real word"; however the verb "digitise" itself is a relatively new addition, and we all know that language evolves through the addition of new words just like this.
In fact, "undigitise" (or "undigitize") is beginning to gain some traction, although so far it tends to be used to refer to reducing one's reliance on technology rather than converting something from digital to analogue.
In a context where digitize means "quantize and record digital samples of a continuous-valued signal", the verb for going the other direction is to reproduce the signal from the samples. I wouldn't say it fills in the blank in the subject-line of your question, but it's often the word that fits in actual usage.
I believe the word you are looking for is "render". It has a variety of applications mostly having to do with converting something into something else or with the creation of art or graphic design either in physical or virtual form.
It doesn't matter what the analogue source is (sound, image or text) when using the word digitise (British English) or digitize (American English) however going the other way (from digital to analogue), the words used would depend on the resulting analogue output.
For images, text, or 3D object, it would be print
For audio it would be sound
verb (used without object)
to make or emit a sound.
to give forth a sound as a call or summons:
The bugle sounded as the troops advanced.
verb (used with object)
to cause to make or emit a sound: to sound a bell.
to give forth (a sound):
The oboe sounded an A.
Similar to this answer (undigitise), I'd use
de-digitise or it's z-equivalent, de-digitize
Examples of similar constructions are compress / decompress and escalate / de-escalate.
To critique your question, I don't think "digitize" is an acceptable verb since it's ambiguous and non-technical. The word discretize would be more apt, since what you're actually describing is an arbitrary function or process which takes values from the discrete to the continuous. And to perform that process, we estimate a continuous range of values between two known discrete points.
In short, the word you're looking for is interpolate.
In the mathematical field of numerical analysis, interpolation is a method of constructing new data points within the range of a discrete set of known data points.
Digital is to digitise, as analogue is to smooth - to transform or modify (a graph, distribution, or function) so as to make it smooth; to lessen irregularities or fluctuations in (something that can be represented by a graph)(OED).
The OP asks
What am I doing when I convert something from digital to analogue? (i.e. when I produce continuously variable physical quantities (OED) from a series of discrete measurements).
'Smoothing', by some form of interpolation, describes in lay terms 'what I am doing' - making continuously variable physical quantities from a series of discrete measurments.
Analogize. You are making an "analog" of the original source by way of a digital sampling.
You might be talking about moving from one storage\playback protocol\format to another.
e.g Convert a
.mp3 music file (compressed, lossy) to
.wav (bitmap, uncompressed, lossless)
In that case you would say you were
converting it to another format\protocol.
VLC, for example, uses the term
An analog-to-digital converter digitizes, but what a digital-to-analog converter does is, it converts.
This is assuming that what you want to capture in your analog medium is something which is already in digital format. If it's not, then what you want to do is to simply "record" it.
When I take a digital video or sound source and convert it to analog (for example by using a tape recorder to record an MP3) I am making an analog recording of the digital source.
In earlier decades, recording a sound almost always meant making an analog recording by magnetic tape. For example, I might record a meeting with a tape recorder, or have a tape recording of a phone message on my answering machine.
When digital recording started to become affordable and mainstream, the word digital was added to indicate that digital techniques were being used instead of the default analog (and of course for marketing reasons.)
"Hey check out my new digital voice recorder, I can record a whole day of classes without having to change a tape!"
Fast forward to today, and digital recording is standard, while analog is the unusual special case. Today, recording is assumed to be digital, unless you specify otherwise by saying it's an analog recording.
In most everyday situations it depends on the medium.
My photo camera collects light and digitizes the amount at each pixel to obtain a picture file. To go back to analog I display the picture. My video camera does something similar, but I play back the video. The sound wave digitized to an audio file (whether compressed or not, lossy or lossless doesn't matter) gets played back, too. The sheet of paper digitized by your fax machine gets printed by my fax machine. The geographic position digitized by my GPS device can be marked on a map, or perhaps I want to visualize a whole track. The position of my stylus on my digitizer board (or your touch device) tends to stay digital, although it might well be used to manipulate things that find their way back into the analog world.
But even for these media, the words above are not truly the opposite of digitize. For each of them, a more common term would be the more obvious counterpart. You take a picture, record a video or audio, scan a sheet of paper, and perhaps log GPS positions. So if people use digitize instead of these, they do so for a reason. Here are some reasons I can imagine:
to encompass different media.
In this case I think render or reproduce would be most fitting. Both of these terms can mean other things, so they will require context or explanation.
to sound technically up to date.
In this case you have to make a decision. You can either use one of the terms I just suggested, without additional context, and hope people will understand what you mean. You can provide more context, thus making your statement longer. Or you can use the more established everiday terms, and risk sounding
to focus on the specific properties of a digitizing operation.
Here the mathematical aspects mentioned in other answers come into play. Perhaps you are worried about the loss of detail due to the digitization process, and want to counter that using interpolation? But then your real problem is more specifically with discretization (usually in the domain, e.g. finitely many distinct pixels) and quantization (usually in the range, e.g. finitely many different color values). So again it's not an exact pair of antonyms.
to describe something without physical representation.
I believe there are people who would use the term digitize when ripping an already digital DVD to a file on their computer. In that case, “digital” is (imprecisely) used to mean something without physical manifestation. You don't have a disc any more, and your computer looks the same as it did before you got the file. In this situation, a proper opposite would focus on the material aspects of the analog representation. You might call your printout (of a scanned sheet or a picture you took) a hardcopy, but I can't think of a verb for this.
One possibility would be
modeled after catalog (catalogue) and log, each a verb.
I also like the answer analogize given by DoritoStyle (modeled on humanize etc.).
The virtue of these possibilities is that they fit an established pattern.
A drawback may be that they already have established usages.
Of course my answer proposes a usage rather than identify one.
Some of the other answers seem to describe what happens in a digital-to-analog conversion rather than name the process.