Wednesday, January 28, 2009

BNC cable impedance

From http://www.canford.co.uk/Technical/BNCConnectors.aspx

Impedance

In its original incarnation, the BNC was designed for RF applications and had a characteristic impedance of 50 ohms, together with a predictable reflection factor. Increasing use in applications such as video equipment, with source and load impedance of 75 ohm and with 75 ohm cables, led inevitably to the creation of the 75 ohm BNC.

At frequencies of up to 10MHz, the characteristic impedance of the connector is completely swamped by the source and load impedance of the circuit of which it is part, thus in analogue video applications it is simply irrelevant whether a 50ohm or a 75ohm connector is used. However the increasing likelihood that the connector is being fitted in a digital video application means that it is absolutely essential that 75ohm connectors and cable are used. This requirement is becoming ever more important as the introduction of HDTV requires significantly higher bit rates so the demands on the cable and connector infrastructure increases.

Tuesday, January 27, 2009

iSight to take quick photos of notes

Need to store sketches digitally
  1. Graphics tablet expensive (though prices have come down), need to trace over drawing, need flat surface
  2. Digital camera - good, but need to take out camera, then connect to computer and download image
  3. Bingo! Use built in iSight to take digital snaps of sketches and store in research manager!
Also, note that PhotoBooth mirrors the view when it shows it to you...

Monday, January 26, 2009

Free Stuff

-------- Original Message --------
Subject: Free stuff
Date: Tue, 1 Jul 2008 10:01:15 -0400
From: XXX
Reply-To: XXX
To: XXX

Hello All,

There are free lab supplies and equipment outside room XXX. Please help yourselves. Also please do not take the cart.

Regards,

XXX

Saturday, January 17, 2009

Homeopathic potency (dosage) nomenclature

From herbdatanz:
Prefix/Suffix Meaning and remarks
TM Ø Tinctura Mater (Mother Tincture) Ø = Theta which is the 8th letter of the Greek alphabet and denotes a 1:10 dilution. The orthodox abbreviation for a tincture is Tr.
D or X Dilutions made on the decimal scale i.e., 1:10. The 'D' is taken from the metric prefix meaning one tenth (1/10). The 'X' is taken from the Roman numeral meaning 10 and was introduced by Constantine Hering.
C or c The 'C' was taken from the Roman numeral meaning 100 and the lower case 'c' from the metric scale meaning one hundredth (1/100). See Table 5-16A.
CM Roman numerals i.e., C = 100 M = 1000
Homeopathic meaning 1:100,000 One hundredth millesimal.
LM sometimes
as 0/1 , 0/2, etc.
Roman numerals again i.e., L =50 LM = 50,000
Homeopathic meaning 1:50,000 The 50th millesimal.
MM Roman numerals, Homeopathic meaning 1:1000,000 Thousandth millesimal
M Millesimal meaning 1:000
H Hahnemann's method of potentisation
K A method of potentisation which was introduced by a Russian General by the name of von Korsakoff in 1829, probably as a labor saving method. However, it produces inacuracies against the Hahnemannian method, so a question mark hangs over it.

Dealing with an MBR virus

  1. fdisk /mbr and fixmbr will fix the MBR but not necessarily get rid of any viruses in it [msoft].
  2. Use the Ultimate Boot CD which has a bunch of low level HD tools from various manufacturers to zero fill the drive, erasing the MBR and any other possible hiding places for the virus. I used the iso found at mrbass.org.
Diagnosis:
I decided that my parents computer was infected by a boot sector virus because:
  1. Installation of Linux or Windows XP went fine from CD.
  2. After the first reboot win xp or linux would stop working and freeze during bootup
  3. I could get winxp working again by booting into recovery mode and using fixmbr
  4. A scan by the seagate tools did not uncover any errors in the disk

Fix the MBR in Windows XP

Boot from the windows XP CD, press the "R" key in the setup in order to start the restoration console. Select your windows XP installation from the list, and enter the administrator password.
Enter the command: "FIXMBR" (without the quotes) at the input prompt and confirm the next question with a "Y" (without the quotes). Use exit to restore the computer.

Friday, January 16, 2009

How to find the SMTP Mail Server for an Email Address

From here:
  1. Open a DOS Command Prompt
  2. Type "nslookup".
  3. Your computer's DNS Server name and IP address will be displayed.
  4. Type "set type=mx" - This will cause NSLOOKUP to only return what are known as MX (Mail eXchange) records from the DNS servers.
  5. For an example, type "hotmail.com" or use your own domain name.
  6. Results returned should look similar to this:

Server: ns1.xxx.com

Address: 192.168.0.1

Non-authoritative answer:

hotmail.com MX preference = 5, mail exchanger = mx2.hotmail.com

hotmail.com MX preference = 5, mail exchanger = mx3.hotmail.com

hotmail.com MX preference = 5, mail exchanger = mx4.hotmail.com

hotmail.com MX preference = 5, mail exchanger = mx1.hotmail.com



Setting up a wiki at HMS

Go to https://wiki.med.harvard.edu/Main/HMSWikiHosting, read the instructions there and fill out a request here http://ritg.med.harvard.edu/support/

Wednesday, January 14, 2009

Caller IDs Indian and US systems

I bought a land line phone with caller ID in the US and tried to have it work in Kolkata (India). The caller ID function does not work. It turns out that the US and India use different methods to transmit the caller id information [1] and you need a converter [2] to get the US phone to work on the Indian system.

[1] In the Netherlands, Saudi Arabia and India, the caller ID standard is DTMF. In the US it is the Bellcore FSK standard and in Germany it is the ETSI FSK standard.
The US also transmits both the user name and the phone number whereas the Indian system transmits just the phone number [artech].

[2] A DTMF to FSK converter is needed. Artech manufactures these calling them Caller ID Converter, DTMF to FSK. The model number is EX230.

Sunday, January 4, 2009

Transit visas at London Heathrow

There is a certain amount of confusion about the conditions under which a transit visa is required when changing planes at Heathrow. There is an official list of conditions here. It seems clear that if you are on the list but you have a visa that allows you entry into the US then you can go in either direction via Heathrow without needing a transit visa provided you don't change airports. It becomes murky when you have a status change when in the US. I flew a week ago via Heathrow and did not need a transit visa.

The following facts may all be relevant or useful to other people:
  1. I had an unexpired (but invalid) F-1 visa in my passport.
  2. I no longer have a valid I-20
  3. I changed status from F-1 to H-1B while within the US
  4. I did not have a H-1B stamp in my passport
  5. My stopover time was 2 hours (as a side note, I would not recommend a shorter transit time because you have to go through security again)
  6. My connecting flight was from the same terminal and the same airline
  7. The only person who checked my passport for visas was the BA desk clerk at Logan
  8. At Heathrow the passport is only used to verify identity
  9. No one asked for an I-20 (I recall vaguely someone asked for an I-20 at Heathrow last time I came to the US via London, but I don't recall if they actually looked at it or merely reminded me that I would need it in the US)
  10. I did NOT need a transit visa