Downsampling

In yesterday’s post 192kHz? I explained how we wound up with the different sample rates.  A convoluted story at best.  Today I thought it might be interesting if I touched on a little known problem with pretty much all CD’s.  Downsampling.

Nearly all master digital studio recordings are made at higher sample rates and bit depths than a CD can handle, even if it’s only 48kHz.  For example, Keith Johnson at Reference records all their materials at 176.4kHz/24 bits and nearly every recording studio is making masters at 96kHz/24 or higher.  When a recording engineer is making a master recording of a group, a copy of a master analog tape etc. he is naturally going to make that master at the best possible resolution he can.  Not at the lowest needed.

So what happens when we want to go make a CD version of that master tape?  The original master is downsampled to 44.1kHz/16 bits through a sample rate converter.  Depending on what the engineer started with, downsampling clearly loses some of what was in the original recording in the process.

How much of what we recognize as inferior playback of the CD, relative to high rez media, is due to this downsampling process?

This practice, I am convinced, is one reason many CD’s sound worse than their high resolution counterparts.  If you read any of the thoughts by recording engineers on this practice, there are many that believe recording at higher rates and downsampling sounds better than keeping everything at the intended use rates.  And then there are those that prefer upsampling.  Clearly, there’s not much in the way of consensus.

In a conversation with Keith Johnson (whose ears I trust completely) he told me the single worst thing that ever happened to any of his work was the downsampling of the master tapes to CD’s.  He hated the results and I am sure this must have prompted their subsequent release of their HRx series with exact copies of the masters.  I’ll bet our friend Cookie Marenco of Blue Coast might have similar misgivings about downsampling.

The difference between the CD and HRx version played on a PWT is obvious and immediate.  But I don’t believe it has as much to do with the increase in dynamic range and frequency response afforded by the higher resolution format as it does by the lack of downsampling.

Downsampling may be one of the worst things to ever happen to the CD and one of the central reasons we judge it so harshly.

It may also partially answer why we can make a digital copy of an analog event without much in the way of loss.  We don’t have to mess with the final result of the conversion, just play it back at whatever sample and bit rate we recorded it at.

We never downsample.

Paul McGowan – PS Audio, Intl.

192kHz?

Have you ever wondered why there are so many different sample rates and why?  The compact disc samples at 44.1kHz/16 bits.  But then there’s these other rates like 48kHz, 88.2kHz, 96kHz, 176.4kHz, 192kHz.  Wow, that’s a lot and the spacing doesn’t seem to make a lot of sense.  How did we get to these numbers anyway?

The first thing to understand is there are basically only two main sample frequencies: 44.1kHz and 48kHz.  Everything else is a variant of one of those two numbers.  96kHz is a doubling of 48kHz, while 192kHz is again doubling that number.  88.2kHz and 176.4kHz are doubled numbers of the original CD standard of 44.1kHz.  But why do we have two numbers, 44.1kHz and 48kHz as the standards?  After all, these are pretty darned close to each other.  Doesn’t make much sense.  You’re right.

There’s a lot of opinions on how we got two primary sample rates so closely spaced together and here’s the best I can make out the history.  It’s an interesting story filled with a lot of twists and turns and we have space for only a few of them.

Back in the late 1970′s and early 1980′s, when digital audio began, most of the recorders were all video based.  I can remember owning a Sony PCM F1 which I used for some live audio recordings.  These rotating head video recorders ran (if memory serves correctly) at 44.1kHz (based on the highest sample rates they could handle at the time) through an adapter made by Sony and at the time were the fastest machines around.  I do recall some debate about using VHS vs. Sony’s Betamax for recording and there was a flurry of opinions and machines one could use.

Then came DAT (Digital Audio Tape also known as R-DAT), which was actually a small and convenient digital audio tape recorder by Sony (among others) that also used the rotating head video recording method to capture audio.  About the same time another variant of DAT called Soundstream digital came along for pro applications – the first use commercially was by Telarc.  Both of these machines were able to record at a slightly higher sample rate than 44.1kHz, the DAT at 48kHz and the Soundstream at 50kHz.

Simultaneously, engineering organizations and manufacturers were both trying to set standards for the new digital audio mediums.  The venerable Audio Engineering Society (AES) in concert with the European Broadcasting Union (EBU) were busy publishing standards that would allow digital audio to be sent without issue between different manufacturer’s machines.  Out of all the is work we got what we now know as the AES/EBU standard (balanced XLR) and the S/PDIF (Sony Philips Digital Interconnect Format) through a single RCA coax or optical cable.  Both formats very close to the same and both handling 44.1kHz and 48kHz plus any variant over those two frequencies.

While all this was going on, the professional recording and film studios were also getting interested in this new digital audio format for their own purposes and most of these pro applications gravitated towards the top end of the DAT sample rate possible, which was 48kHz.

So here we had a situation where Sony and Philips were pushing hard on the 44.1kHz Compact Disc standards and the pro and film markets were happy with a slightly higher sample rate of 48kHz.  It is widely rumored that the Compact Disc 44.1kHz standard evolved in order to allow a double album’s worth of music to be placed on the small silver disc.  One story even suggesting the head of Sony demanded Beethoven’s entire 9th symphony fit onto the small disc (about the same length as a double album).  Others debunk both stories as absurd since all the engineers would have to do to fit this much data with 48kHz is make the CD larger – about the size of a 45 rpm vinyl disc – and then the Sony marketing people wanting to have no relationship in size, color or look of anything vinyl making the decision.  Best I can tell it was a simple holdover from the original PCM video recorders which couldn’t handle anything higher than 44.1kHz/16 bits (the minimum required for 20Hz to 20kHz reproduction).

On the pro and film side the now common place 48kHz sample rate, a defacto “standard”, appeared on a growing number of pro-side tools and equipment and used without a lot of question.  Popular lore had it that the film industry used (and still uses as the DVD standard) 48kHz to “synch” to the standard film rate of 24fps (frames per second), but as I think about this I cannot even envision how this would matter at all – being just a convenient doubling of 24.  Synchronizing the audio to the video is essential, but the relationships between 24fps and 48kHz doesn’t seem to actually have any merit – other than that’s what everyone does.

So, in the end, we wind up with two very similar yet different primary sample rates, 44/1kHz and 48kHz.  From these two numbers come all other PCM sample rates and that, my friends, is why we have 6 types of sample rates on DACS today.  Three would be just fine, but because of the double standards of 44.1 and 48, we have 6.  Go figure.

Paul McGowan – PS Audio, Intl.