Posts Tagged ‘dB’
Similar to the the Pleasurize Music! project I mentioned a while, turnmeup.org is a website dedicated to increasing the dynamic range of new albums. Their goals are stated as: 1) Defining an objective measure of dynamic range on a record, 2) defining a level of dynamics that is considerably more dynamic than today’s agressively limited records, but not so quiet that it wouldn’t be an option for contemporary artists, and 3) establishing and putting into place a system to measure and certify records that would like to be considered for Turn Me Up! certification.
They’re letting people join the organization as supporters, submit albums to be certified, and become mastering houses capable or certifying records. This looks like an interesting project, and Vagrant Records (among others) is one of their corporate sponsors; I just hope this all leads somewhere!
Peter Kirn at Create Digital Music did a post on the Pleasure Music Foundation back in March. The Foundation aims to put the pleasure back in music – pleasurize music! – by advocating for greater dynamic range in music. From their Aim:
Our aim is to improve the sound quality of music in its various recorded formats – including data compression methods such as MP3 – as well as music destined for radio broadcast.
Only music that provides a positive musical listening experience has real market value. The Foundation’s aim is to increase the value of music within the creative production process for the entire music industry.
The objective is to revive the willingness to pay for music and therefore to create a healthier basis for all creative participants within the music industry.
They’ve produced a Dynamic Range Meter that gives every recording a score, a function of the peak amplitude and rms level (though this is really the crest factor of the music, not the dynamic range). The meter is available for all platforms.
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 20*log10(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, 20*log10(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 20*log10(4/1) = 12.04dB. This is the value of a dB scale – we can add gain values in dB instead of multiplying them: 2*2=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 = V*V/R, where R is the resistance the power is dissipated through. For instance, if I drive 1 Volt into 1000 Ohms of resistance, 1V*1V/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 2V*2V/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, 10*log10(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(P*R). This is still an absolute measurement, so you need to know what the input signal was to calculate the dB out: 20*log10(Output/Input).