Monty, not quite.
First, I’ll clarify where the ‘circle’ comes into play in Japanese. There are two 48 characters syllabaries in Japanese. That’s 46 plus 2 odd ones you’re not likely to see written in modern Japanese. There are however far more syllables in the language; 89 in total (88 if ‘wo’ is simply pronounced ‘o’ as is usually the case).
Japanese uses two different symbols to change the basic syllables. These marks are usually put a little over and to the right of the character. The first symbol is called dakuten (voice mark), which look like two elongated dots. For example, the ‘k’ series, “ka ki ku ke ko”, becomes “ga gi gu ge go” (the vowels stand for their latin equivalent. Correspondingly, ‘s’ becomes ‘z’ and ‘t’ becomes ‘d’. There are exceptions, but who really wants to know? We’d be up to 48 + (3*5) = 63 syllables, but it’s actually 61 because of complicated phonetic reasons that you probably don’t want to hear.
The second symbol, the handakuten (half voice mark), which is a small circle, is used only for the ‘h’ series. If you add it to the series “ha hi hu (a.k.a. fu) he ho” you get the “pa pi pu pe po” series. If you replace the handakuten by the dakuten you get the “ba bi bu be bo” series. Now we’re up to 61 + (2*5)= 71 syllables.
The remaining 18 syllables are made by combining the syllables for ‘ki’, ‘shi’, ‘chi’, ‘hi’, ‘mi’, and ‘ni’ with the syllables for ‘ya’, ‘yi’, and ‘yo’ to obtain syllables like ‘kyo’ in Tôkyô.
Now, concerning the double consonnants. The symbol used is a smaller version of the syllable ‘tsu’ put before the syllable whose consonnant is doubled. I can WAG why this character was chosen if you’re masochistic enough to ask me. Let’s take Nissan as an example. It is written in Japanese as ni-(small ‘tsu’)-sa-n. That’s four characters, the final ‘n’ being one character.
What it actually sounds like in Japanese is another matter. Most people will agree that it makes the ‘s’ sound stronger and/or longer. Personaly, I find that it also shortens the previous vowel. It clips it, so to speak.
Momotaro, the Peach Boy
Only humans commit inhuman acts.