Hi Dom,
Re the laptop-to-laptop and laptop-to-desktop testing.
Dom wrote:
I'd be interested to know how you get on with the laptop to laptop tests. I have been doing something similar here last week - to test scrape / player were working - and I found that the line-in on my main machine (supposedly good quality) sound card had quite a high frequency for its low frequency cut off (around 100Hz ish by the looks of the waveforms) which led me a merry dance - NBTV syncs just don't work well in such circumstances. I then tried an el-cheapo chinese usb sound card I had and that worked excellently.
Interesting that the cheapo usb soundcard worked so well, I have a couple here and will try them out. Regarding the poor low frequency response of some sound cards, this topic cropped-up in an exchange with Steve Anderson a few weeks back. Steve had a couple of tips for getting the best low frequency response and to save my typing fingers I will quote him directly...
Steve Anderson wrote:
As I recall (but don't quote me) the output coupling caps on my HP desktops were 100uF, with a 32 ohm load that's a -3db point of 50Hz - pretty crappy even for music let alone NBTV. They usually have a series resistor of a similar value to protect against short-circuits on the output. This brings the -3db point to a more respectable 25Hz or so. When the load is 1k or greater that internal resistor can be forgotten about. I tend to go for a 10k minimum input impedance to NBTV stuff followed by a unity gain (Av +1) op-amp (though a non-inverting op-amp with gain will be OK) or a transistor emitter-follower.
While on the subject of connecting laptop-to-laptop and laptop-to-desktop I should just like to throw in a cautionary note. Many experiments have been performed here using various soundcard-to-soundcard and soundcard to radio combinations mostly related to amateur radio digital modes and/or SDR and one problem crops-up quite frequently when using laptops (and sometimes desktops as well) in which an AC pottential difference exists between the two machines being interconnected. This can happen for a number of reasons but in my experience the two most common causes are transformer leakage and/or the decoupling capacitors used to reduce RFI in the SMPSU's.
At best this AC Voltage difference can cause unwanted "noise" to appear at the input of the soundcard and at worst it can give you a nasty "belt" if you happen to touch the laptop/desk top and the interconnecting audio cable. While the "belt" has never been what I would call life threatening its enough to be anoying and my feeling is that it could also cause damage to the solid state components in one or both machines. In one particularly bad example I had a look on a scope and found the noise was over 50 V pk-to-pk and was a combination of 100 Hz and around 60 kHz from the switching supply of the laptop in question.
We radio hams normally avoid this noise/pottenial difference issue by using an audio transformer and this is fine for typical 300 to 3000 Hz frequency response of the average communications tranceiver but for NBTV or SDR radio it would need to be a very high grade transformer. I found a better (and much cheaper) solution is the use of a cheap "generic" opto-coupler which offers total isolation of any pottential differences between the two items of equipment and also offers a supperior frequency response.
The reason I "waffle on" about this is because I have encountered this problem yet again while looking at coupling two laptops together for nbsc testing. Indeed, I got quite a "tingle" while lifting one of the laptops over the bench while holding the audio lead in my hand. So I am currently in the process of cobbling together a revised opto coupler with extended freqency response for nbtv use. I may be wrong but I would imagine a fairly flat gain/phase relationship would be desirable for nbsc in order to prevent spurious phase changes of the colour subcarrier components?
Thats planned for the w/end so I will let you know how it goes. BTW I tested "scrape" in combination with the output of the webcam as displayed on the laptop screen and had the audio output routed to the loudspeakers. Using the slightly higher performance laptop I heard no signs of choppy audio but will hook-up a CRO to the output at the w/end and take a closer look since the human ear can be quite tollerant of shorts breaks in audio.
Cheers for now,
Des.