Jens, I wasn't sure what you intended. Might have been Russian "thermometer" tubes as display even. I wonder what the various guys who have mentioned making audio meters ended up doing?
Even though you want it for a headphone level indicator, it might get used by others for different purposes. The problem with any averaging meter in today's audio environment is the fact that digital systems clip when overdriven. I mentioned tape recording [analogue] earlier. Tape has a natural compression effect [I said limiter last time]. It was the lack of realisation of this that caused so much of the argument when digital workstations and digital storage arrived. The tape recording produced more punch because they couldn't afford to operate down in the linear region. [Signal/noise]. A VU meter needs an audio system with 14 to 18dB headroom. Because the meter is slow it does not indicate peaks. Taking voice intelligibility into account [some clipping allowed], voice peaks are generally 4 to 14dB above what is indicated. A PPM can be used simply - "don't go into the red". The zero [ie start of red section] indicates that clipping is occurring. As I mentioned before 0dBFS should be a little down from the red [big topic]. FS means Full scale in the digital arena - the biggest number that can be represented by your 16 or 24 bits (or whatever). For your headphone use you could make the red zone be the onset of distortion caused by the headphones overdriving OR it could represent the max safe listening level [assuming you don't have level limited phones already]. Sound card specs used to give good levels info. I just looked at my motherboard handbooks from the last 15 years and they have zilch. I am mentioning all of this because if you go to the trouble of coding something then you have the opportunity to build in features. Another reason for going a bit overboard is this: I believe that when shortcuts and liberties are taken with standards it should be with the knowledge of what has been done. It should be decision based - not a result of ignorance. I am a bit wary of pointing to a wiki article, but you have probably already been there.... https://en.wikipedia.org/wiki/Line_level An aside: now that we are talking audio, I would love to get an argument going about Watts RMS ! I say this: the watts specified for an amplifier output are intended to be average power and not rms power. When Watts rms is stated it is NOT intended to be read as that. It is intended to mean "Watts [rms derived"]. There is a lot of difference. Any takers? I invite sceptics to do the math or draw a sinewave power waveform and inspect it carefully. Try 1V rms and 1A rms sine wave resistive. Having looked at it and tipped the top half into the trough [proving what I proposed], go ahead and perform an rms calculation on that power waveform. You might be surprised. If that didn't get your attention, look at it his way. Even the respected name companies use the W RMS term. So, how can I be right? John K Australia -----Original Message----- From: neonixie-l@googlegroups.com [mailto:neonixie-l@googlegroups.com] On Behalf Of jb-electronics Sent: Friday, 29 March 2019 03:09 To: neonixie-l@googlegroups.com Subject: Re: [neonixie-l] OT: audio levels Hi David, hi John, Thank you very much for your help! I am not trying to build a professional VU meter; rather, what I want is to create a microcontroller-based LM3916 alternative. These are now obsolete LED bar driver ICs with a logarithmic output that are quite handy for building a simple (not professional) VU meter. Lots of things to read for me! Best wishes Jens On 2019-03-28 8:10 a.m., johnk wrote: > Hey Jens... > > 1). Don't call it VU unless you match the actual ballistics of the > ORIGINAL VU meter > > 2). Europe tends to use a PPM [quasi-peak] system. Peak Program Meter. > > 3). Consumer and pro levels are very different. They are often misunderstood > and hence mounds of garbage appear on the web. > > 4). dB is a ratio and a reference must be stated. Thus you must choose what > reference your 0VU is. > > 5). Your headphone out MIGHT be the Line Out. There are [quite a few] > standards for Line Level. > > 6]. A useful fact relates to the maximum level that can be digitally encoded. > A CD can produce a certain maximum digital value. That gets converted to some > analogue value. You can work empirically by using [say] standard test tones > from the web OR an audio program like Audacity to produce known digital > levels. 0 dBFS is the max level... BUT there can be output results higher > than this [long topic], so work on using either -0.3dB or even -3dB below (or > do I mean over? Semantics ) this as maximum. > > 7). In "recent" years the "loudness wars" and misuse of Digital Audio > Workstations has resulted in CDs and digital audio having bugger-all dynamic > range. Magnetic tape was automatically a limiter [curved]. People using > digital should be working [say]20dB down from max.... but the signal level > looks so tiny to them in "oscilloscope" mode. They think that they are > getting better results by working "hot" and getting the best sig/noise > ratios. Their loss of dynamic range is not a good outcome. I mention this > because it DOES relate to the choice of 0 dB[ref]. > BTW.. some digital audio actually clips! The master has too much dynamic > range for the "cutting" [vinyl, cd , web etc] engineer. He wants it to sound > HOTTER. Sigh. > > 8). Talking the voltage levels. The term dBU refers to a bridging voltage > measurement. These days you definitely do NOT want to try expecting 600 ohm > impedances. The "usual" outputs are these days exceptionally low... eg way > under 20 ohms. This way you can feed a great number of inputs from one > output. Big topic. The problems start here for your question. Many consumer > outputs place a series resistor for protection purposes. This doesn't affect > Bridging (high Z meter) readings much, BUT the input impedance of the loads > does. And the number of such loads connected. > There are many manufacturers who conform to certain industry standards, and > many more who don't. > > 9). Read Bob Katz book Mastering Audio. Downloadable, although I didn't check > which editions. I use 2nd edit for recording because I do not do web stuff. > His later edition(s) cover the "modern" approach. [read Amateur Fiddling for > modern :-)) ] > > 10). I suspect that the most useful meter for your purposes is one which > relates well to the maximum signal level and has fast response. BUT VU is so > averaged and hence slow. There are many so-called VU that are fast reacting > and hence display peaks. And be careful of the definitions here when you see > rms stated. As David said, work with peak voltages that you can see on a > scope. This is where you need to start looking at the Orban meters. I > haven't checked but they were downloadable. Apart from the damping factor for > Peak, his special meters that relate to perception are interesting. He covers > the standards for TV advertising versus program content. And that leads to > discussions of loudness [not volume]. > Reminder... VU can ONLY be used when the meter has the defined ballistics. > See the BTSJ papers from the 1940s iirc. And the original meter HAD/HAS to > have the attenuator incorporated. The use of the meter without recognising > the lack of sensitivity led to the actual zero level of +4dBm. The original > switched attenuator had zero attenuation in the 4 position. It takes +4dBm to > drive the meter to the zero indication. Big topic too. Watch out for web > misinformation. You can check my version by referring to the papers. I can > not [easily] access all my digital archives at present so I am not providing > full details. > > 11). As David implied... I think you will need a sensitivity adjustment. > Users can calibrate the zero to represent whatever they want. The meter > characteristic/ballistic HAS to be known by a serious user. There are > examples on the web. > Whatever you do has to provide for the whole audio chain.. there has to be > headroom everywhere so that there is no clipping. > > > 12). Here is a typical online statement - this one form Audio Technica > :- > > "Line level refers to the typical level (strength or amplitude) of the audio > signal from mixers, signal-processing equipment and other consumer and > professional audio gear. There are two line level types: consumer and > professional. Consumer line level is generally thought of as a signal whose > level is at -10 dBV (0.316). CD players and DVD players are examples of > consumer line level equipment. Professional line level is generally thought > of as a signal whose level is at +4 dBu (1.23 volts or significantly higher). > Signal-processing equipment and professional mixing consoles are examples of > professional line level equipment." > > > Done quickly, apologies for errors and omissions, John Kaesehagen > Australia > > > > -----Original Message----- > From: neonixie-l@googlegroups.com [mailto:neonixie-l@googlegroups.com] > On Behalf Of jb-electronics > Sent: Thursday, 28 March 2019 11:42 > To: neonixie-l@googlegroups.com > Subject: [neonixie-l] OT: audio levels > > Hello Nixie friends, > > I am thinking of building a simple all-in-one VU meter using a > microcontroller to visualize the audio signal on a headphone line (say, I > feed the headphone output of my PC to my circuit, and then I install a > pass-thru to connect to my headphones). > > What is the typical voltage amplitude I should expect? I do realize this > question is quite general, but perhaps you can point me in the right > direction? Many thanks! > > Best wishes > Jens > > -- > You received this message because you are subscribed to the Google Groups > "neonixie-l" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to neonixie-l+unsubscr...@googlegroups.com. > To post to this group, send an email to neonixie-l@googlegroups.com. > To view this discussion on the web, visit > https://groups.google.com/d/msgid/neonixie-l/2a4dfbef-7430-06ac-b272-973d84c6dae8%40jb-electronics.de. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to neonixie-l+unsubscr...@googlegroups.com. To post to this group, send an email to neonixie-l@googlegroups.com. 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