2 questions - volume and channels

[EddieG]

First question:

Why is volume in decibles instead of the old scale of 1-whatever? And why does volume start in the negative numbers? How can you hear a negative decible? Isn't zero no sound at all?

And what exaclty is a channel, as in 150 watts, 30 watts per channel? Please explain in laymens terms

Thanks!

Eddie

 

[MDS]

The volume display is a scale - a range between min and max. Nothing more.

The absolute scale is what most people are used to: 1 - 100 (for example). The relative scale is the one where the numbers start negative and go slightly above zero: -80 - +20. There is no difference between a scale of 1 to 100 and a scale of -80 to +20 as they both represent 100 steps between minimum and maximum.

With relative volume display, 0 is the 'reference' and all other numbers are relative to it, hence the name relative volume display. When you calibrate levels you use 0 as the value that yields a given SPL. So if for example you calibrate to Dolby Reference Levels, when the display reads 0, the SPL will have a max of 105 dB. When the volume display reads -10, the SPL is 10 dB below your reference or 95 dB.

With the absolute scale you do exactly the same thing but it isn't as convenient. For example, my reciever goes from 0 - 80. I set the reference level at 60. If the volume is currently at 50, I am at 10 dB below reference (just like the above case with relative display), but you have to calculate: my ref is 60, I'm at 50, 60 - 50 = 10, therefore I am 10 dB below reference level. With the relative display, it shows directly that the level is 10 dB below reference.

 

[Haoleb]

And what exaclty is a channel, as in 150 watts, 30 watts per channel? Please explain in laymens terms

Thanks!

Eddie

A channel in laymans terms is for example what drives a single speaker. Such as Left channel (left speaker) Center Channel (center speaker) etc.. Watts per channel refers to the amplifier's output power into each channel.

 

[OttoMatic]

The volume scale represents the amount of attenuation (or gain) applied to the preamp-level signal. At "0" there is no gain applied to the preamp signal, and the amp is running at its nominal gain.

At -3dB, you have a applied 3 dB of attenuation to the signal, and it's therefore quieter. The more attenuation you apply, the quieter it gets.

Consider separating the preamp from the amplifier (in the case of a receiver). You plug your CD player into your preamp, and your preamp to your amp. Set the gain to 0 dB. In theory, and if everything's implemented properly, the preamp should have no effect on the level of the signal, and your amp will be fed a fairly large signal (it'll be loud). If you take the preamp out of the equation, and plug the CD player directly into the amplifier, you should get the same preamp level signal to the amplifier, and you should get the same loudness (SPL) out of your speakers. Either way, with the CD player into the preamp (and volume set at 0), or with the CD player plugged directly into the amplifier, there should be no attenuation on the CD player's preamp level signal.

So, the more "negative" you go, the more you are reducing (attenuating) the preamp level signal. It's reducing it from its nominal value, so you can still "hear" a negative volume.

Similarly, if you go positive, it's simply indicating that the preamp level signal is being increased from its nominal value. Feeding that larger preamp level signal to the amplifier (or the amplifier section of your receiver) will give you a louder signal at the speakers.

MDS is right in that many people use the 0 dB level as their reference level. However, the reference level chosen is somewhat arbitrary. Many people use 75 dB SPL (as measured in the room with an SPL meter) as their target reference, and 75 dB in the room will not necessarily correlate to 0 dB on the volume knob, because 75 dB in the room is dependent on the gain of the amplifier and the sensitivity of the speakers, among other things.

 

[MDS]

The volume scale represents the amount of attenuation (or gain) applied to the preamp-level signal. At "0" there is no gain applied to the preamp signal, and the amp is running at its nominal gain.

At -3dB, you have a applied 3 dB of attenuation to the signal, and it's therefore quieter. The more attenuation you apply, the quieter it gets.

Not when we are talking about the volume control or the numbers on the display. If the scale is -80 to +20 there is zero attenuation at +20 (it's the max). 0 is roughly 80% of max.

If you were talking about the bass or treble control then 0 is the mid-point as they usually have a range like -10 to +10. In that case 0 is no gain, -10 is 10 dB attenuation and +10 is 10 dB boost. The same scale would apply to a balance control or a fader.

MDS is right in that many people use the 0 dB level as their reference level. However, the reference level chosen is somewhat arbitrary. Many people use 75 dB SPL (as measured in the room with an SPL meter) as their target reference, and 75 dB in the room will not necessarily correlate to 0 dB on the volume knob, because 75 dB in the room is dependent on the gain of the amplifier and the sensitivity of the speakers, among other things.

Yes you can choose any SPL as your 'reference'. A reference is just a number to which every other number is relative. My 'reference' home temp is 75 degrees. If the current temp is 72 degrees, it is 3 degrees below reference. The thermostat could use the same scale as the relative volume display of a receiver and read '-3 degrees' instead of showing the absolute temp of 72 degrees.

When you calibrate a receiver's levels you make it so that 0 (or whatever point on the dial you want to use) results in 75 dB. Room size, speaker sensitivity and all that stuff affects the SPL but that is exactly why you have the channel trims to adjust things to reach your target.

 

[OttoMatic]

Not when we are talking about the volume control or the numbers on the display. If the scale is -80 to +20 there is zero attenuation at +20 (it's the max). 0 is roughly 80% of max.

I'm very sure that 0 dB on the display means that the preamp signal is at unity gain (well, within the limits of the preamp anyway -- there may be a little roll off or other less-than-perfect artifacts, but if everything were perfect, it would be at unity gain). There are measurement and discussion about this type of thing here, and I spent quite a bit of time measuring an Outlaw 990, as well as discussing the results and working to figure the "right" way (or at least a "good" way) to measure and represent these things.

Although I was measuring a preamp, the results for a receiver should be generally the same. Measure the input signal and compare it to the signal at the preamp outs. Should be unity gain.

When you calibrate a receiver's levels you make it so that 0 (or whatever point on the dial you want to use) results in 75 dB. Room size, speaker sensitivity and all that stuff affects the SPL but that is exactly why you have the channel trims to adjust things to reach your target.

You can do it either way, and neither way is wrong. If I calibrate to 75 dB SPL and my preamp is at -3 dB, then that's my "reference level."

 

[MDS]

I'm very sure that 0 dB on the display means that the preamp signal is at unity gain (well, within the limits of the preamp anyway -- there may be a little roll off or other less-than-perfect artifacts, but if everything were perfect, it would be at unity gain).

Your measurements may have produced that result but you've drawn the wrong conclusion from it.

Some Onkyo receivers allows you to switch between absolute and relative display and 0 on the relative scale is equivalent to 82 on the absolute display. There is no magic to the relative volume display.

The volume display is a scale - nothing more - as I've explained no less than 2 dozen times. The scale could be labeled A, B, C....ZZZ and it would be the same. It could be labeled A-Z and then 1 - 74 if the total range was 100 steps and it would be the same.

A volume control is an attenuator as you've correctly pointed out but a receiver or amplifier has a fixed rail voltage and the volume control simply attenuates that voltage. At the minimum end of the range the attenuation is full, ie mute; at the maximum end, the attenuation is zero. It does not matter what letter or number is on the display - each point is just one point on the scale between minimum and maximum and the relative display is convenient for the reasons I've stated.

Unity gain is a concept that applies to tone controls, faders, mixers, and things like that. Here is a non-audio example of the concept of a relative scale and unity gain:

My sprinkler system has a feature called 'seasonal adjust' that allows you to change the run time of each station without changing the program. The scale is 10 to 150 in increments of 10. 100 would be 'unity gain'; ie if the station is set to run for 30 minutes and seasonal adjust is set to 100 then the station runs for 30 minutes. In other words 'no change' or unity gain - just like a treble or bass control set to 0 (because 0 is the middle of the scale). If seasonal adjust is set to 50, the station runs for 15 minutes; if set to 150 it runs for 45 minutes. That scale could just as easily have been -9, -8, -7, ...0, 1, 2, 3, 4, 5 - like the relative display of the receiver.

It's really not as complicated as many people would like to think.

 

[OttoMatic]

Some Onkyo receivers allows you to switch between absolute and relative display and 0 on the relative scale is equivalent to 82 on the absolute display. There is no magic to the relative volume display.

That makes sense, though, right? I bet they've implemented 0 dB to be unity gain, as they should. Then, sure, they can assign any arbitrary "other" scale to the volume. There's no magic there.

The volume display is a scale - nothing more - as I've explained no less than 2 dozen times. The scale could be labeled A, B, C....ZZZ and it would be the same. It could be labeled A-Z and then 1 - 74 if the total range was 100 steps and it would be the same.

I agree that, in general, it can be anything. It's when they assign a "dB" value to the display that it explicitly means something -- it means that it's x dB away from a reference, and from an engineering point of view, that reference is the incomig signal.

A volume control is an attenuator as you've correctly pointed out but a receiver or amplifier has a fixed rail voltage and the volume control simply attenuates that voltage.

The volume control most certainly does not vary the rail voltage; it varies the preamp level signal.

At the minimum end of the range the attenuation is full, ie mute; at the maximum end, the attenuation is zero.

Not necessarily. It can also apply gain -- when the decibel volume scale reads positive, the preamp/receiver has applied gain to the preamp level signal.

It does not matter what letter or number is on the display - each point is just one point on the scale between minimum and maximum and the relative display is convenient for the reasons I've stated.

I disagree. Furthermore, I'd also suggest that each increment on a "dB" volume scale represents one decibel of signal power inside the machine itself (if you measured the signal at the preamp level or in the power amp), as well as one decibel of measurable SPL in the room.

Of course, I can't say the same for any "other" volume scale that doesn't have "dB" attached to it. One increment on that type of scale could be anything -- one dB of change or more or less.

Unity gain is a concept that applies to tone controls, faders, mixers, and things like that.

Unity gain is a concept that applies to any filter, including a volume control or other gain device -- it's just that the bandwidth of such controls is infinite (or wide enough to handle the entire frequency spectrum of the device in question; for this discussion it's probably to assume that 10 Hz to 20kHz could be considered "infinite"). If I set the volume control, regardless of the numbers displayed, such that the input of the audio signal (a CD player, say) is the same at the output, then there is unity gain applied (i.e., neither attenuation nor gain).

It's really not as complicated as many people would like to think.

I agree that it's not complicated. In fact, the concept that 0 dB on the volume knob is unity gain is so patently simple that it makes perfect sense. Sure, other volume "scales" (e.g., 0 to 100, or 1 to 74, or anything else that doesn't have a "dB" behind it) can be whatever they want; but once it has a "dB" attached to the number is specifically means something. It is relative to something, and it's relative to the incoming signal; it's not arbitrary once they associate "dB" with it (unless it's improperly implemented).

 

[MDS]

Ok Otto, so where is unity gain on a receiver that only uses an absolute scale?

The volume control most certainly affects the rail voltage level. The channel trims boost or attenuate the pre-amp level.

The volume numbers do not 'have a dB attached to them'. I can boost the volume of a waveform in Sound Forge by 3 dB. Now how does it do that if it has to be related to an input signal from a pre-amp? It's a scaling operation which means multiplication and the volume control does the same thing but to voltages instead of sample values.

 

[OttoMatic]

Ok Otto, so where is unity gain on a receiver that only uses an absolute scale?

I'm not necessarily sure what you mean by "absolute scale," but if you mean one that uses an arbitrary scale (e.g., 0 to 100 or 1 to 50 or whatever, without any "dB" indicator attached to it), then I would have to say "I don't know." I couldn't say without measuring it. It's there somewhere, but without having a reference (i.e., "0 dB") one simply can't be sure.

That case would be similar to the implementation of an analog volume control -- one that associates no scale or numbers with its output level (e.g., the old Kenwood receiver that I use in my basement; analog volume only). There's unity gain on it at some point, and I'd have to measure the input to the system (e.g., from a CD player) and then measure the output at the preamp outputs. When input == output, it's unity gain. Then I could say something like 11:30 on the volume knob is unity gain.

Similarly, if I measure an arbitrary volume scale at the point where input == output, I could say 73 on that volume scale is unity gain. Then, if we were able to implement a dB scale on that same system (like you said is available on the Onkyo), 73 on the arbitrary scale should be the same output level as 0 dB (and both are unity gain).

The volume control most certainly affects the rail voltage level. The channel trims boost or attenuate the pre-amp level.

Rail voltage? Why would you want to affect rail voltage with a volume knob? Rail voltage is where your amplifier section is going to clip. Perhaps there's some rare exception or a specially implemented power supply that would something like that (covering all bases here), but I'm sure that most normal preamps and receivers won't vary rail voltage via the volume knob. Perhaps you could explain how varying rail voltage is going to affect volume.

The volume numbers do not 'have a dB attached to them'.

I thought that's what we're talking about. If I look at my preamp right now, it says "-42 dB." That's what I'm talking about.

I can boost the volume of a waveform in Sound Forge by 3 dB. Now how does it do that if it has to be related to an input signal from a pre-amp?It's a scaling operation which means multiplication and the volume control does the same thing but to voltages instead of sample values.

Not sure I understand. Sound Forge and your preamp are two different thing. I'm talking about an input to the signal to the preamp compared to the preamp output level of the preamp.

A volume control can also affect samples (a digital volume control), but I don't think most preamps are implemented that way.