Martin's audio & video problem-solving page

a selection of postings to various user forums

I've been a regular contributor to user groups and technical forums for the last 20 years or so, and have written hundreds of posts offering information and advice on a whole range of computer, audio & video production topics. Trouble is, I never kept copies of any of them, which is why I've created this page. Martin Kay - May 2010 PS Maybe I should just call the page "Martin's Sound Advice", since most of the current posts are about audio.

Q    Sweetening an Echo? Have a bit of audio that has turned out a bit on the echo'y side.... ....and it is a little unsatisfactory. Are there any way in which we can tidy or sweeten this a little bit by using either a filter in FCP 7 or within Soundtrack Pro or Audacity?  Would appreciate any comments and advice from those in the know. Thanks in advance. Alex

A    Martin writes...

If this is "colouration" from unwanted reflected sound then generally, once added, it is impossible to remove. Sometimes it can be improved, but it depends on the nature of the echo, how it's been generated and the frequency spectrum it occupies.
 
If you have what might be called "boomy" echo, where much of the energy is in one or more narrow frequency bands, as might be caused by pronounced resonances in an enclosed space (where each fundamental resonant wavelength is the distance between two parallel walls), then this can be improved by careful frequency-based filtering (eg a parametric EQ or notch filter). Reducing the energy in these narrow bands will remove some of the wanted sound, but remove a bigger overall proportion of the unwanted sound, if indeed most of it is concentrated in narrow frequency bands. However, if the echo is much "cleaner", with a nice even spectral distribution that matches the wanted sound, then frequency-based filtering is not going to help, as in every instance you remove the same proportion of wanted and unwanted sound.

To use a parametric/notch filter to reduce resonances, the technique I suggest you try is first to set the filter to boost rather then cut, and then slowly sweep it through the frequency spectrum until you hit one of these resonances in the recording, at which point the sound will get far worse than before as the resonance is exaggerated. When that happens, try and tune-in the centre frequency to where it sounds the most horrible, then switch the filter to cut, and adjust the amount of reduction to taste. If there is a Q Factor adjustment, this will control the width of the frequency band affected, either side of the main/centre frequency. In general, use as narrow a band as you can get away with so as not to affect the wanted sound too much. However, there is no magic "one size fits all" setting for this sort of work, so let your ears be the judge of what works best for a particular recording (or part thereof, if the mic was moving and catching different resonances in different parts of a room.

Q    Stuck with a Red Line in Premiere Pro CS5.5 I have imported a lot of still images into my sequence and by adjusting the duration of each I have created a flick book animation effect. There is now a red line above all of these new/adjusted images in the timeline and I understand that this means some rendering is required. However, when I go into the Sequence pull down menu and select both render options - 'Render effects in work area' and 'Render entire work area' - nothing happens!? The Red line remains and the viewing bar flicks to the very beginning of my sequence and begins playing (both times) This is confusing and frustrating but I've every faith that one/several of you can suggest an alternative/easy fix.

A    Martin writes...

The bottom line is:- does it play OK? If it does, then why worry? The yellow line indicates that the playback engine expects real-time playback. The red line means it doesn't expect real-time playback, but that doesn't mean it won't happen (on a system with a fast CPU). The issue is further complicated if you have an I/O card like those from Matrox or Blackmagic, whose drivers will sometimes cause the red line to be set in anticipation that real-time playback is not guaranteed, but may nevertheless happen.

Based on previous versions with red/green lines (and no yellow), you may not feel comfortable unless there is no red showing, but CS5 is different, and unless you're doing a play-out to tape (where a dropped frame in a red-line segment would be an issue) there's nothing to worry about, unless there's so many dropped frames that the preview is un-watchable. Your exports will be fine, as everything's effectively getting rendered at that stage anyway.

Q    Desk Microphone - Recommendations? I'm currently recording a voiceover in Final Cut Pro using a lapel microphone. Obviously this isn't an ideal situation. Can anyone recommend a decent priced, good quality desk microphone, so we can have a permanent set-up to record voiceovers. I'm a bit of a novice when it comes to recording audio. If I was to set something up in a quiet room, what would I record on to? Apologies if this seems a daft question.

A    Martin writes...

You could always plug it into your camera? Assuming it will take an external input. If not, budget in a very basic mixer like a Behringer Xenyx 502 which can feed into the line-in of your PC, via a long cable.

Here's a check-list for recording good speech.

1) A reasonably "dead" room, acoustically, so it doesn't sound like you're in a room when the pictures might be of an exterior scene. Avoid small, empty rooms with bare walls and lots of hard surfaces. The more soft furniture, thick curtains, cushions and general clutter, the better. If needs be, temporarily hang something like a duvet across the room to damp down any resonances.

2) A reasonably quiet environment. It's pretty obvious you don't want extraneous sounds.

3) Any half-decent microphone. In my experience, a mediocre microphone in a good environment will produce a more usable recording than the world's best microphone in a poor environment (ie one with bad acoustics). Also, in that context, avoid placing the mic on a desk stand, as the desk is a hard, reflective surface. Better to stand up and use a floor stand (or even hand-hold the mic, with care) so that it's in "free air", about a foot in front of you. I don't know what your budget is, but there's a lot of relatively low-cost large-diaphram mics that are intended for studio vocals recording, like the Behringer C1/C3 and Samson C01 models, JoeMeek JM47, Rode NT1a, etc. Buy the best you can afford, and it'll probably last you a lifetime. Most of my best mics are over 25 years old.

4) Record at as high a level as possible without clipping! If it clips (and distorts) you can't fix it in the edit - just record it again. If it's way too low in recorded level then you compromise the signal to noise ratio, and you can get quantisation distortion from not using enough bits in the digital range.

Q    What is the File size / Fps / Resolution dependency? Hello,  The problem I've encountered goes beyond my logic. I work in Sony Vegas Studio Platinum. I render a clip using the h.264 codec - once with 24FPS and the other time 12FPS (half film). Logic tells me that if the number of frames has been halved, the file size should also considerably decrease whereas it remains almost the same. If my memory serves me, I haven't also noticed any considerable decrease in file size when I halved the resolution. The only factor having much to say was bitrate. Why is that so? If you could cast some light on the subject question..

A    Martin writes...

The answer is pretty much in the question, where it's stated that "The only factor having much to say was bitrate". Bit-rate is exactly that, the rate at which the data (in bits) is generated (per second) by the codec. For example, if the bit-rate is 480Kb/s (four hundred and eighty thousand bits per second), and the video was 20 seconds long, then the file size would be 20x480Kb (= 9600Kb). There are eight bits (small 'b') to a byte (big 'B'), and file sizes are normally expressed in bytes (or kilo-bytes KB, or mega-bytes MB), so the value of 9600 kilobits is divided by eight to get a value in bytes. 9600 / 8 = 1,200KB or 1.2MB.

It is the bit-rate and duration that dictate the file size, nothing else. So what effect do you get from changing the frame rate or pixel resolution? Well, there is another equation which links those things to bit-rate, along the lines of:- Pixel Resolution x Frame-rate x Relative Quality = Bit-rate. If you want to increase the values of any of the items on the left (Pixel Resolution, Frame-rate or Relative Quality), you should increase the Bit-rate by the corresponding percentage. If the bit-rate stays the same, then increasing one of the other values (Pixel Resolution or Frame-rate) will cause a decrease in quality from the codec, and visa-versa.

Of course some codecs are better than others, particularly newer ones like H264, so simply using a different compression codec and keeping the other values the same will change the relative quality of the encoded video. Strictly speaking, the "Relative Quality" element is really "Relative Quality/Codec Efficiency", such that a more efficient codec will produce a particular subjective quality at a lower bit-rate than a less-efficient codec. However, if you're already using the best codec available for the job, it makes the equation simpler to leave it as it stands.

FYI - This is the sort of information that's being covered at the IOV VideoSkills "Better Encoding" training events     

Q    How can I improve the quality, particularly volume, of a recorded piece of audio?? Hi All,  I have a problem that I need some 'sound advice' with! Filmed a Wedding and had two mics capturing the audio. One lavalier style mic on the groom and another set up to catch the reading. It appears that their was a slight issue with the lavalier mic and it has not recorded most of the vows! Aargh! However, the other mic (Sennheiser ME64 with K6 power unit) was working well and has picked up some of the missing audio. However it is extremely quiet and not really passable. In my limited experience all I can do is boost the volume but this gives unsatisfactory results. Does anyone have suitable experience or know any people/companies that specialise in audio who might be able to help clean up some of this or at least help lift it slightly?

A    Martin writes...

Yes, I specialise in audio and, from what you describe, the phrase that comes to mind includes the words "cat" & "hell" and general negative connotations.

You don't describe in what way your attempt to lift the volume was unsatisfactory, but I can guess. If the sound you're left with is from the ME64 which was set up for the reading, then it was in the wrong place and probably not pointing in the direction of the vows. That gives you three problems.

1) The recording is low level. If that was your only problem, then you could simply lift the volume to the right level. It might bring up some electronic noise with it, but you might be able to reduce that with software noise filtering and EQ.

2) If the mic is "off-axis", then it will have an uneven frequency response to sounds coming from that direction, which can be difficult to correct, given the complex EQ that would be required, and the fact that any EQ you apply will affect the noise as well as the wanted sound.

3) If the mic is "in the wrong place" and further away from the source, it will pick up a lot of diffused (reflected) sound, particularly from the direction which is "on axis", to which the microphone is most sensitive. These combined effects are known as "colouration" (if it was light rather than sound, it would be the equivalent of colour fringing and colour casts on the image). To remove it, you're up against the physical laws of entropy, and in my experience it's a non-starter. EQ won't help, and there's no "un-reverb" effect that I know of. It is probably possible to enhance it for forensic purposes (ie simply being able to hear what's being said), but it still won't be a nice sounding recording - more like the comms sound on the Apollo space mission.

If it was possible to get a good recording from putting a microphone in the wrong place and fixing it in software, no-one would bother to do it properly and boom operators would no longer be required on dramas, for example, but I don't see that happening yet.

Those, sadly, are the audio facts of life, in my experience.

Q    How to overlay a minutes & seconds counter? On one of my projects, the customer has requested a running time showing just minutes and seconds on the bottom left of the screen as it is a training video. I have a plug-in for superimposing full Timecode of Hours:Minutes:Seconds:Frames but the brief says just Minutes and Seconds is enough! Does anyone know if there is available a Plug-in for this, or a Time Code Plug-in where you can choose what parts of the clock to display? Although it is our plans to change over to Final Cut Studio in the next few months, we are still running three suites with Avid Liquid Edition 7 and they are still working very well. Any help will be much appreciated.

A    Martin writes...

Could you not just crop it to include the part of the timecode display that you require?

Or you if can't apply the crop to the timecode effect (and I'm assuming you do have a "crop" effect?), make two copies of the footage, one on top of the other, apply the timecode to the top one, and then crop away everything apart from the desired digits to reveal the other copy underneath. I've done that in Premiere with two versions of the same sequence.

Just for the record, this procedure will work in real-time in Premiere CS5 - no intermediate rendering or "fusing" required - so I would be hopeful of the latest Final Cut having similar capabilities.

Premiere allows whole timelines (or as they call them, Sequences) to be interactively nested into other sequences, hence it's very easy to add an effect to an entire sequence, and to have a second version of it (with a different effect plus a crop/mask) on that same timeline.

Q    Sound balance with two mics into my camera? As a 'newbie' I have a Sony HVR HD100E camera which has a 3.5mm mic input that the on-board mic connects to (No other)). I also have a Sony UPW V1 lavaliere mic receiver on the camera. To use them simultaneously and following helpful advice, I obtained a 'two-to-one' cable that enabled both sound inputs. But, unless I am missing something obvious, no matter what I do, the sound obtained from the lavaliere dominates the input from the on-board mic, despite putting the mic in front of a radio with the input from the lavaliere coming from another room!! When played back etc., the on-board sound is barely audible. I would appreciate any suggestions, even if 'tough'!!

A    Martin writes...

Two thoughts on this.

1) Unless you're using the most rudimentary editing software, you can adjust the individual levels of each channel in post. I think even Premiere Elements can do this through the Channel Volume effect, which is certainly present in every version of Premiere Pro.

2) If the track with the lower volume (the on-board mic) is really too low to boost, then you need to attenuate the other input so that both can be recorded at a decent level. This can be done with two resistors, costing pence, some wire and and a soldering iron. You may have to pay someone to do this, but it's not going to cost a fortune. As to what values the resistors should be (i.e. how much attenuation should be produced), you should look at the audio meters in your (audio or video) editing software and see what the actual difference is (in dBs) between the channels, or how much gain you have to add to the lower one to match the higher one. From that an engineer can easily work out what values to use. (This is a service that I can provide)

3D - a view from the sidelines posted to the IOV Knowledge Base 3D forum, May 2011 Let me start by saying that I've not exactly been "wowed" by the whole 3D bandwagon, but I am warming to it. I've been helping one of customers, Ian Sandall of SPL, get his Adobe CS5 edit system set up with Neo3D and the Nvidia 3D Vision Pro active-shutter glasses to be able to view the Premiere timeline in full-screen 3D on an external LCD monitor. It's been quite a learning curve, with both hardware and software issues and niggles to overcome, but when it's all working it can look very impressive. And seeing it in my edit suite, not at a cinema theatre or a video show, gave me more time to think about how well 3D might work for various applications. Previously, I'd been to see Alice in Wonderland at a 3D IMAX theatre. I thought some of it worked really well, particularly the 3D CGI "wonderland" elements, but some of the non-CGI shots were less impressive, resembling layers of 2D cardboards cut-outs (what I think of as the "Captain Pugwash" effect, for those over a certain age). My conclusion was that even with a Hollywood mega-budget they weren't getting it right all of the time, so what chance would the rest of us have, particularly if we weren't using CGI but simply shooting live action? At the IOV Show in October 2010 I had a look and a play with the Panasonic AG-3DA1 and once again wasn't terribly impressed. But that's probably because I didn't have any meaningful tests that I wanted to try, and when some new piece of technology is simply sitting on a stand for people to experiment with, it can be difficult to know what to do to really bring out the best in it. Pointing it at various poorly lit random objects whilst wearing 3D spex to view the wall-mounted 3D monitor does not make for a great test, which combined with the high price left me somewhat under whelmed. So where's the positives then? Well, some of the 3D test footage on Ian's system was from Neil Richards, produced using his AG-3DA1. Shot in and around a churchyard, it's very unpretentious, non-Hollywood style footage, that probably any of us could have shot. And does 3D bring anything to the viewing experience that 2D doesn't? Yes it does. A real sense of depth and scale that you don't get in 2D. Did every shot look better in 3D? No it didn't. Decisions over framing in 3D, particularly with regards to foreground objects, are more critical than in 2D, or else you can get "orphaned" fragments floating around the edge of frame which appear in front of the screen, disconnected from anything in the rest of the scene. But the potential was clearly visible. Back home, watching the Chelsea Flower Show in HD, I was thinking, "I bet this would look good in 3D". Likewise, it would be interesting to see anything architectural in 3D, from Grand Designs to The Country House Revealed. And would this not relate to IOV members producing promos involving similar subject matter, like a country house hotel or golf course? I think the simplest criteria for deciding whether 3D is worth considering for a video, aside from cost and how the audience will view it, is based on whether you have control over the shooting schedule or not. It can take time and planning to set up a good 3D shot, so where you have time and the opportunity for re-takes it will pay dividends. Where you have less control, particularly at a live event, 3D is an added complication which seems hard to justify. As for costs, I see from a Google search that 3D cameras can be hired for around ?40/3days for the consumer HDC SDT750, or ?120 for the 3DA1, should you want to experiment. On the editing side (for Premiere CS5x) you need the Cineform Neo or Neo3D plug-in (recently reduced from $2999 to $999) plus, if you want the best viewing experience, a "3D-ready" 120Hz monitor, a set of Nvidia 3D Vision Pro active-shutter glasses and an Nvidia Quadro card to drive them (FX3800 or 4000 are the cheapest), all of which would probably add up to around ?1000-1200 ex-VAT. Or you could start out just using Neo with anaglyph glasses for around ?200 ex-VAT (assuming you've already got a 2nd monitor for the preview o/p) and upgrade the rest of it later. You may have noticed that I've made no mention of "distribution", like 3D BluRay disks, which is largely because we haven't got to that stage yet! It's also because, as far as I can see, support for the MVC 3D encoding format is only available in a few (expensive) high-end products, but there are other lower-end solutions like side-by-side which are far easier to implement, albeit with a few caveats. I'm guessing this may the subject of a future IOV article at some point in the future...

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