Category Archives: Discrete Music

+4 dBu / -10 dBV

+4 dBu / -10 dBV:  These values are typically used to represent ‘Pro’ and ‘Consumer’ audio signal levels - some gear even has a switch to scale the output, or to adjust input gain.  So what do these values mean on a common scale?

First we need to know how to get from dB to volts.  For linear measures like volts, we defined the measure in dB to be 20log10(volts/reference).  To go backwards we’ll need volts = reference10^(dB/20).

You’ll recall from a previous post that the reference for dBV is 1 volt, and the reference for dBu is 0.775 volts.

Let’s convert 4dBu to dBV, via volts:

0.775*10^(4/20) = 1.228V

20*log10(1.228/1) = 1.8dBV

So +4dBu=+1.8dBV, and the difference between +4dBu and -10dBV is really about 12 dB.

Blu-Spec CD

At the end of last year Sony announced a new audio disc, the Blu-Spec CD.  This is an interesting idea - they’re using a blue laser (the same process as Blu-Ray Disc) to burn a standard Red Book Audio CD.

To be clear, this is not a high-res audio disc.  It’s a standard Red Book CD, playable by all CD players.  The difference is in the creation process.

The advantage is that the shorter wavelength of the blue laser allows for more precision in the pitting of the disc.  This should result in fewer errors on playback when read by a standard CD player with a red laser.  This raises an interesting question about the audibility of errors in CD playback - what do they sound like, and how well does error-correction work?

Pro Tools – Shortcut of the Week (2009.06.08)

Pro Tools is a strong multi-track editing program, but unless you’re willing to drop some change on a custom keyboard, it can be tough to learn all the keyboard shortcuts.  I’m featuring one a week in an attempt to highlight the tricks I find most useful.

Bounce to Disk:  Command + Option + B

There’s a debate with Pro Tools about the quality of the summing bus - many people claim to prefer the sound of summing through an Aux channel over the Bounce to Disk tool.  When I’ve finished a mix I usually record the mix onto a separate channel, but before that point I find myself using Bounce to Disk quite a bit.  I probably use it most when playing out takes for an artist, for take selection.  If I have a lot of takes to work through, clicking File->Bounce->Bounce to Disk gets tedious.  This shortcut is a huge time saver when I’m working in this mode.

dBV, dBu, and dBm

I’ve always have a little trouble remembering the relationship between dBm and the more common signal measurements of dBu and dBV.  The other day I was looking at a transfer function, and I was given results in dBm - the impetus for this post.

First, I should make sure we’re all on the same page.  What is a decibel (dB)?  The decibel is a relative measurement.  For voltage we calculate dB as 20log10(Voltage/Reference), where the reference can be absolute (like with dBu and dBV) or relative (plain old dB).  For an amplifier that outputs a signal twice as large as the input, 20log10(2/1) = 6 .02 dB.  If we ran a signal through this amplifier twice, our output is 4 times as large as the input, or 20log10(4/1) = 12.04dB.  This is the value of a dB scale - we can add gain values in dB instead of multiplying them: 22=4, while 6.02+6.02=12.04.

Before going on I should point out that dBm is not an appropriate measure for a transfer function.  Straight dB is the way to go, as a linear transfer function should produce an output relative to the input - not relative to an absolute value like dBm, dBu, or dBV.  Let’s say I put 0.5 volts into a system, and I get 0.25 volts back out: 20*log10(0.25/0.5) = -6 dB.

So then dBV, dBu, and dBm are all absolute measurements and each has a defined reference value:

  • dBV - 1 Volt
  • dBu - 0.775 Volts
  • dBm - 1 Milliwatt

dBV is straight ahead and simple to work with - this is the measure I use most frequently.  The dBV value of any voltage is 20*log10(V/1), so 1 Volt is 0 dBV, 2 Volts is 6 dBV, and 0.5 Volts is -6 dBV.

Let’s skip to dBm for a second.  This has a reference of 1 milliwatt, a unit of power.  Power is related to voltage by the equation P = VV/R, where R is the resistance the power is dissipated through.  For instance, if I drive 1 Volt into 1000 Ohms of resistance, 1V1V/1000Ohms = 0.001 Watts = 1 mW.  If I increase the voltage by 6 dB (a factor of 2) to 2 Volts, now I’m dissipating 2V2V/1000Ohms = 4 mW.  This is important - doubling the voltage does not lead to a doubling of power, since the power is related to the square of the voltage.  We actually have to use a different formula for dB of power, 10log10(Power/Reference).  With this formula, if we put in our 4mW and 1mW from above, we still get 6 dB of gain.  Math works!

Back to dBu.  dBu is referenced to 0.775 Volts - an odd number to use as a reference.  However, if you calculate the power dissipating by 0dBu through 600Ohms (a long-standing common value for transmission lines), you wind up with exactly 1mW of power, 0dBm.  So 0dBu = 0dBm, if and only if the load is 600 Ohms.  At any other load impedance this relationship changes.

So back to that transfer function in dBm.  In order to get a relative dB measurement I needed two things: the source voltage and the load impedance.  Knowing the load impedance allows you to calculate the voltage output from the system: V = sqrt(PR).  This is still an absolute measurement, so you need to know what the input signal was to calculate the dB out: 20log10(Output/Input).

Pro Tools – Shortcut of the Week (2009.06.01)

Pro Tools is a strong multi-track editing program, but unless you’re willing to drop some change on a custom keyboard, it can be tough to learn all the keyboard shortcuts.  I’m featuring one a week in an attempt to highlight the tricks I find most useful.

Zoomer Tool:  Command+1

Trimmer Tool:  Command+2

Selector Tool:  Command+3

Grabber Tool:  Command+4

Scrubber Tool:  Command+5

Pencil Tool:  Command+6

Smart Tool:  Command+7

Seven shortcuts!?!  While normally I think that this would be too many to digest in one shot, it wouldn’t make any sense to list these separately.  These tools are lined up across the toolbar in the order above, with the exception of the Smart Tool.  I think it would be harder to remember the shortcuts above if you didn’t see them in one place.

The Smart Tool is actually sitting under the Trimmer/Selector/Grabber tools, but I think it’s easy enough to remember that as command+7 - we read left to right, top to bottom.

So why bother with these shortcuts?  Why not just use the Smart Tool all the time?  I’m just making the transition away from the Smart Tool myself.  The Smart Tool is easy - no need to remember which tool does what, but it also doesn’t have much depth.  Among other behaviors, command-clicking changes functions with each tool.  This means more control with fewer wrist movements, and that’s the main purpose of shortcuts, right?

Note: You can also use the F5 through F10 keys to access the six individual tools, but I work on laptop keyboard primarily.  The function keys are all dedicated to other tasks, and I’m happy with this setup right now.

Pro Tools – Shortcut of the Week (2009.05.25)

Pro Tools is a strong multi-track editing program, but unless you’re willing to drop some change on a custom keyboard, it can be tough to learn all the keyboard shortcuts.  I’m featuring one a week in an attempt to highlight the tricks I find most useful.

Extend Selection to Previous Region Boundary:  Option+Shift+Tab

This is the converse of last week’s shortcut, Extend Selection to Next Region Boundary.  While I don’t use this shortcut all that much, the principle is the key takeaway.  Adding Option to most commands in Pro Tools will either do the opposite action (instead of adding a marker, it might remove a marker) or reverse the direction of the action, like in this case.

Pro Tools - Shortcut of the Week (2009.05.18)

Pro Tools is a strong multi-track editing program, but unless you’re willing to drop some change on a custom keyboard, it can be tough to learn all the keyboard shortcuts.  I’m featuring one a week in an attempt to highlight the tricks I find most useful.

Extend Selection to Next Region Boundary:  Shift+Tab

This is perfect for grabbing a selection when you’ve either already trimmed the region boundaries, or if the boundaries are just close enough.  I always play out the tracks I’ve recorded for the artist to listen to before I start a mix.  This lets them catch any last minute issues before I devote a lot of time to a project.  Since I always through down markers to tag the starts of takes, this shortcut is an easy way to grab a whole take, quickly.

Mortal Music Recording

Last weekend I was over at Mortal Music Recording Studios in Charlestown, MA.  Andy Pinkham runs the space, and he’s put together a great room with some great toys. 

We were recording Boston band Pray for Mojo, a four-piece instrumental group.  The drum kit was miced up in the live room, DI off the bass and keyboards, and the guitar amp was isolated in another room for flexibility later.  Everything came out really well, and I’m looking forward to getting back over there sometime soon.

Links:

Mortal Music

Pray for Mojo

Castle Recording Studio

This past weekend Belikos asked me to come out to Utica, NY, to help with a recording session at Castle.  The band was solid, and cranked out most of two complete tracks in one day.  I’m planning to do most of the post work from home.

Castle was a very cool place to track.  Matthew Wagner, chief engineer, is a great guy and found some fantastic sounds.  The room is an old church, and designs for the conversion were done by our old friends at the Walter-Storyk Design Group (the same folks that redesigned the Duderstadt Center Audio Studio at the University of Michigan).  I sensed that it was a John Storyk room when I walked in, based on the design and acoustics - very good all around.  

My only complaints were the decay time in the live room (useless for drum tracking - and why else have a live room???) and the Yamaha DM2000 console.  From what Matt said, they’re still working on the decay in the live room, so they get a pass - I’ll be curious to see what they’ve done next time I’m back.  The console though, I don’t know about.  If all of the work you do is in the box, what does a digital console get you that a control service doesn’t?  Especially one as convoluted as a Yamaha board?  I had the experience of learning on a Yamaha O2R96, which is basically the same interface.  Unteachable.  And I sure don’t remember how it works.  Good thing Matt was around.

I’ll post pictures and audio later.  [Castle Recording Studio]

Micro Controller Options

 

Arduino - <i>Photo by Nicholas Zambetti</i>

Arduino - Photo by Nicholas Zambetti

 

Last year I had the opportunity to help out a friend with a project for a class he took at MassArt.  We used a microcontroller with buttons for a UI, and fed the data into the serial port of a computer.  We used Max/MSP to generate MIDI data and finally used Ableton Live to trigger audio sample playback from the MIDI.

The system was overly complex, but the coding was actually a minimal amount of work.  It got me thinking about a few μController-based projects that have been on the back burner for a few years.  I figured it would be good to conduct a quick survey of the chips that are being commonly used and have the best online support networks.

Arduino:

The Arduino is actually what we used for the Max-Ableton project I mentioned above.  This is a family of creative commons controlled boards, with open source software to build on.  My experience working with the Arduino was very enjoyable - the combination of a high-level language and lots of support online made the job easy.  You can get a great board for as little $30, and you’re pretty much ready to go out of the gate.  The boards are based around ATmega RISC micros.

There’s also a great series of articles on producing sound from an Arduino here.

Phidgets:

Phidgets are a system of chips and sensors linked to a computer via USB.  These boards are designed to operate in tandem with the computer, instead of as standalone processors.  There’s an API for pulling together interfacing applications on the computer side, and there’s a Max/MSP object for interfacing directly with the Phidgets.  The sensors available include buttons, encoders, accelerometers, PH sensors, and thermocouples, ranging in price from $28-95.

Handyboard:

Handyboard is an MIT design, with a focus primarily on robotics.  These boards are setup for reading sensors and driving motors.  There are only a few vendors, and the boards run in the $100 range.  There’s a fair amount of information on the MIT site, but the online community using the Arduino platform seems to be a lot larger.

PICMicro:

The PIC platform has been around for years, and is a simple solution for the serious DIYer.  This solution is a little messier than some of the others listed above, because you’re dealing a lot more at the component level and coding in assembly language.  You’ll need a PIC programmer, and then you’re pretty much ready to go.

The biggest benefit I can see from PICs are the low cost of the chips.  With the Arduinos you’re likely buying $30 boards; you can can PIC chips for anywhere from $1-8.  The economy of scale is very nice here, but for building a one-off circuit I’d likely stick to the pricier options above.

Gumstix:

I added this to the list because of the audio capabilities, but this is less a microcontroller system than it is a tiny Linux computer.  The cost is greater than the systems listed above, and the documentation is still under development.  I tried a project on this about a year ago, and ran into a number of issues.  I attribute them to my limited experience with Linux, something I’d like to rectify.

This list is far from complete - it’s just a few of the platforms I know people are working from.  As I gear up for a new project, I’ll continue to look into suitable μController platforms and post about them here.