Previously, previously, previously, previously, previously.
So when do we see this in XScreensaver? :-D
What's neat about guard tones is they recapitulate the history of the development of the phone system. So you get a burst of 150 bps at the beginning (still used for emergency broadcast system on the radio) and probing for filters (the bursts of high frequency as it goes full spectrum).
It's basically path MTU discovery for the audio spectrum.
Is it possible to know what is the "sending" and "receiving" side path of those bands?
See also http://www.windytan.com/2012/11/the-sound-of-dialup-pictured.html which gives more technical detail and does it as a poster.
It's a fair cop. In mitigation, I would like to plead that the "previously" links are not the best way to list relevant previous posts.
Still, it's cool that Oona now offers poster prints of some of her prettier spectrum analyses.
What the fuck do you think the Previously links are?
A perhaps not entirely unreasonable question. They are a series of the words "previously" in a lower-contrast colour which can get tuned out for not appearing to be part of the blog post.
So sometimes I check them out with gusto, hungry for more. Sometimes I fail to even notice them. In this case, it was the latter, for which I can only apologise.