Decibels...dbs.

[murray]

HI to all ......could the users of the forum please help me ..With a general discussion on ..DBS..and how you would use it in an audio restoration....the + and the ... minus ...of using it in a general sense for the uneducated i"m waiting to hear your comments....thanks ...murray.

[Brian]

Back when Craig and I were still in diapers (well me anyway), and telephones were cranked instead of speed dialed a unit of measurement called "miles of loss" was used as a standard in calculating signal levels in telephone networks. A signal transmitted through "a standard-loop mile of No. 19 telephone wire" would suffer a transmission loss that was pretty much the same everywhere. (A loop mile is actually two miles of wire, one going and one returning.) This transmission loss was soon changed to a logarithmic expression, "transmission unit" - which was eventually renamed the "bel" in honor of Alexander Graham Bell. In practical use, the "bel" was too large for most applications so measurements were more commonly found in tenths of a bel and the unit was modified accordingly to a "decibel" - abreviated "dB".

The decibel doesn't have a numerical value but expresses the ratio between two powers, voltages, or currents. (Actually it can express the ratio between many other units of measure such as pressure, impedance, etc.)

In normal use, one of the two powers, voltages, etc. is standardized at a reference level and that reference level gives the decibel a particular name. The most common reference level for a decibel in the broadcasting, recording and electronics industries is one milliwatt (0.001 watt) and is written "0 dBm". So, if the letters dBm appear behind the number zero it means the reference power is 1 milliwatt. If the number is other than zero you can calculate what the actual power level is using the chart on page 441 of your Diamond Cut software manual (or any scientific calculator or computer program). There are many different reference levels used with the dB. I'm not familiar with the reference level for a dBs - is it possible that, in the discussion you mentioned, they were just talking about dB - plural?

Decibels are used in just about every aspect of audio work including restoration, however its relevance has changed with the switch from analog to digital systems. If you want more details, I would highly recommend "Audio: The Movie", an instructional DVD sold for $39 by Tracer Technologies - here's a link:

http://www.tracertek.com/learn-about-audio-dvd

Hope this helps, and welcome to the group. Lurk here and you can learn a lot, particularly from Craig, Rick, Doug, Geebster and literally thousands of other members.

Brian

[Dan McDonald]

Nice explanation, Brian.
Murray, for the general user like me, I think of it in this way: If you're working on restoration and you normalize the signal to 0dB, then you have the full capacity of the system to work with.

That means (to me) that you can listen at a normal level and will be hearing proportionately more of the recorded signal than any noise that your playback system is introducing. If you are at -10 dB in your recorded file, for example, you may hear some hiss in your signal that is due to the playback system and not the file you are playing, so it is a waste of time to try to get rid of that noise - it's not in the file.

If you are doing restoration work that pulls material out of the signal you are restoring, then you will be ok if you have gain-normalized to 0dB. If you are doing restoration work that adds to or modifies the signal, you will risk distorting the signal because the result will be greater than the system can handle (+1 or +2 or maybe higher dB). So, if you'll be modifying the signal, you'll be better off at around -2 or -3 dB for the normalization before you begin work on a file.

Also, in the real world, some playback systems are unable to actually produce the signal at 0dB, so some people prefer to gain-normalize to -1, -2 or -3 dB as the final product.

Anyway.. a nontechnical way of dealing with it.
Dan

[Brian]

Boy did I goof up - I certainly meant to include Dan as one of the top sources in the group - as his post here proves!

The DVD I referenced goes into this in a little more detail and shows how to do it in the DC software - but Dan just saved you $39 plus shipping!

If you're already through the basics, the "advanced" DVD from Tracer which also covers this is "Advanced Forensics Concepts" - a bit newer and also highly recommended.

Brian

[Craig Maier]

Hi Murray,

I have never heard a clearer description of the dB than that provided by Brian. I have nothing to add but for the willingness to address further questions for which Brian will probably provide the most clear of answers. And Dans practical approach is very useful. These guys know what they are talking about.

If you still have further questions, I will take a shot at it if you think it necessary!



Craig

[Dan McDonald]

Thanks Brian. I'm not much of a resource, but I try to help out as I can.

Dan

[Craig Maier]

This does not add much to the discussion, but mathematically, dB is as follows pertaining to a Voltage (or Current) or Power Amplification systems:

dBV = 20 Log VOut/VIn (for Voltage Amplification)

or

dBI = 20 Log IOut/IIn (for Current Amplification)

dBw = 10 Log POut/Pin (for Power Amplification)

As an example, if a preamplifier takes a 10 mV signal on its input and creates from that a 1 Volt signal on its output, the Voltage gain in db would be:

20 Log 1.0 / 0.01 = 40 dB.

If you have a preamp which has a Voltage gain of 40 dB, driving a post amp having a Voltage gain of 20 dB which drives a power amp which has a Voltage gain of 10, the total Voltage gain of the system is simply the sum of the three gains in dB (70 dB) since the dB unit is logarithmic.

You can find more details on this topic in your Diamond Cut Users Manual.