Following the "spelling" surely suggests consistently spelling 電(でん) as "den", not alternating n/m depending on the environment? The Japanese don't write different んs for 電波(でんぱ)・電流(でんりゅう)・電話(でんわ).

Attempting to approximate pronunciation is a valid theory of transcription, but one which also ought to prescribe that 電気(でんき) be transcribed as dengki; English is not much less discerning of syllable-final [n] vs [ŋ] as it is vs [m]. This is not a position I've ever seen anyone defend in earnest, though.

(Romanization for anglophone is a bit of a lost cause anyway, since we're going to fuck up the vowels no matter what you do.)

> Attempting to approximate pronunciation is a valid theory of transcription, but one which also ought to prescribe that 電気(でんき) be transcribed as dengki; English is not much less discerning of syllable-final [n] vs [ŋ] as it is vs [m].

That is blatantly incorrect. English converts syllable-final [n] to [ŋ] when followed by a velar exactly the same way Japanese does, and English spelling reflects that. Consider the English words "think", "clunky", or "handkerchief".

Sure, now show us lack of assimilation to a subsequent bilabial (in a context where /nk/ does assimilate), which is what Japanese does and that you're implying English does differently (it doesn't). English has it baked in so deeply that most would-be /np/s are already spelled <mp>, which muddies the waters a bit, but these past few days have given us plenty of clips of people pronouncing "government", haven't they?

What are you trying to show? You seem to agree that the English spelling of /nt/ is "nt", the English spelling of /ŋk/ is "nk", and the English spelling of /mp/ is "mp". There is no possibility of "np", "nb", or "nm".

How would that suggest that it's reasonable to spell the Japanese word "dempa" as "denpa"?

For demonstrating lack of assimilation of /n/ to following bilabial, there are a couple distinct questions you might ask. It's very frequent for people to preserve the tongue gesture associated with /n/, because a bilabial stop doesn't use the tongue and so [n] is easily coarticulated. But that turns into /mp/ or /mb/ over time because the difference is not easy to hear. In contrast, for a word such as "impossible" where this process completed many hundreds of years ago, the tongue is not used at all in the pronunciation of /mp/. This is a kind of lack of assimilation.

You can also see lack of assimilation in the very people who go to special efforts to pronounce [n] in Japanese words where that is inappropriate.

Note that the English and Japanese phenomena you're talking about are very distinct. This is a fact about the historical development of sounds in English (and Latin...) that doesn't apply to current English, where a sequence like /ng/ will often be preserved across word boundaries. ("One ghost"; this is the only context in which such a sequence can occur at all.[1]) English maintains a robust distinction between /n/ and /m/ and a weaker one between /ŋ/ and the other two.[2]

In contrast, Japanese ん assimilates to whatever follows it, and in the case that nothing follows it it may (rarely) be realized as nothing more than nasalization of the preceding vowel. Word boundaries are not relevant. Japanese does not have a phonemic syllable-final /n/ or /m/ (or /ŋ/). It has a single sound (usually indicated /N/ by specialists, apparently, due to even more weirdnesses that it involves) that gets realized differently in different contexts.

So again - what would justify representing the Japanese sound as "n" regardless of context in languages where, unlike in Japanese, the distinction between "n" and "m" is meaningful?

[1] You say that most would-be /np/s are already spelled "mp", but this is false - the words that are spelled "mp" changed long ago, and do not represent attempts by modern speakers to pronounce an /np/ sequence. They represent attempts to pronounce an /mp/ sequence.

[2] Why weaker? /ŋ/ doesn't have the status the other two do; it cannot begin a syllable. And it makes for a less than perfect contrast with /n/ and /m/ because it has a fairly pronounced effect on the vowel that precedes it, which makes drawing a clean contrast difficult.

> What are you trying to show? You seem to agree that the English spelling of /nt/ is "nt", the English spelling of /ŋk/ is "nk", and the English spelling of /mp/ is "mp". There is no possibility of "np", "nb", or "nm".

Consider "inpainting", "unbiased", and (as suggested earlier) "government", each of which is a synchronically transparent /n/ across a morpheme boundary, yet a cursory survey of recorded English speech suggests that it's pretty common for these tongue gesture associated with /n/ to be absent—infamously, the second syllable of the last routinely loses its coda altogether. This occurs across a transparent morpheme boundary, even with affixes productive in the modern language, even in learned usage.

English does have a lot more wrenches to throw in this, like producing nuclear nasals in a range of situations and not always assimilating across prosodic word boundaries—heck, it probably goes both ways in an utterance like "in my main menu". Words spelled "mp" are reliably [mp] in the modern language, but it's not a simple case as "mp" spelling /mp/ read [mp] and "np" spelling /np/ read [np]; English phonotactics also coerces the nasal in /np/ to a bilabial realization.

> because a bilabial stop doesn't use the tongue and so [n] is easily coarticulated

That doesn't sound quite right—this assimilation surely wouldn't be nearly as globally prevalent as it actually is if that were true.

Try it. While you'd think from the descriptions that a bilabial stop shouldn't care where the tongue goes, I think you'll find it quite challenging to coarticulate [n] with [b]—tongue positioning at lower teeth is pretty obligatory—and much easier to sequence them or produce [mb].

Clearly you can see the unnaturalness of lack of assimilation to call the attempt to do so "special effort"! So of course, the typical anglophone is not going to try to realize [n.p], they'll just see the <np> and read [mp] because that's what they would with any other internal /np/.

> So again - what would justify representing the Japanese sound as "n" regardless of context in languages where, unlike in Japanese, the distinction between "n" and "m" is meaningful?

Now, this gets to an entirely different issue: the purpose of the transcription. You seem convinced that the main goal of romanization is to provide a pronunciation guide for anglophones. But in the context of discussing a niche musical genre on the internet, that's not necessarily a high priority in the first place; you might care more about, say, searchability: we're looking for https://en.wikipedia.org/wiki/Denpa, not https://www.worldbank.org/en/programs/debt-toolkit/dempa.

And in a wider context, the principal users of romaji Japanese aren't anglophones; they're Japanese-speakers who for some or other reason need need to coerce Japanese text into an ~ASCII-subset representation, targeting primarily computer systems with that sort of limitation (most common case being keyboards via IME, hold that thought) and secondarily other people who can read Japanese; and naturally they make the distinctions Japanese makes and largely don't make the distinctions Japanese doesn't make. So unless backed by a marketing department, they tend to produce n (or nn as needed) for ん, because they have a tenuous grasp on how anglos spell [mp] in the first place and でmぱ is garbage that their IME won't convert into the right word, so why type that?

(This is also why pinyin can be the way it is, yet their IMEs have routinely have modes to ignore s-sh/n-ng/n-l distinctions.)

>> because a bilabial stop doesn't use the tongue and so [n] is easily coarticulated

> That doesn't sound quite right—this assimilation surely wouldn't be nearly as globally prevalent as it actually is if that were true.

> Try it.

You know, I mentioned a specific theory here that you've completely ignored. The coarticulation is easy. But it is difficult for a listener to tell the difference between coarticulated [nb] and [mb]. If you're willing to let multiple generations pass, this means that /nb/ will become /mb/ regardless of how easy it is to pronounce.

You will also note that this theory of what's happening mostly cannot be disproved by recordings, which you appear to want to do. You'd want an X-ray or MRI study, something which shows you what the tongue is doing.

> I think you'll find it quite challenging to coarticulate [n] with [b]—tongue positioning at lower teeth is pretty obligatory

This is just obviously false. You have no problems producing [b] with your tongue positioned however you like. You can position it for [t], you can position it for [tʃ], you can position it for [k]. And of the three coarticulations I just mentioned, all of them are well attested, though only the middle one is attested in English ("pshaw", a scoffing sound).

> and much easier to sequence them

This is worthy of comment; there is a linguistic concept called "coarticulation", but all cases of coarticulated consonants seem to have a conventional sequence associated with them. I have no real knowledge or opinion on how real the conventional sequencing is, or how much sequencing is allowed before you stop calling the sounds coarticulated. I suspect that indeed it is easier to sequence two events than to coordinate them to occur at exactly the same time; this is true for all types of events, not just language-related ones. I don't think that the linguistic concept requires absolute synchronization of particular points in time; my understanding is that producing any given phoneme requires some motion and therefore takes place over a nonzero span of time, and "coarticulated" consonants are those for which the durations overlap, not necessarily those for which the durations perfectly coincide.

But I will note that while sequencing of /nb/ is obviously necessary in a way that is not true for /pt/, since /n/ must have nasal airflow and /b/ must not, there is no reason for "coarticulation" of /nb/ to be more difficult than it is in the attested coarticulation /tm/ (exactly the as /np/ for our purposes; /tm/ also features a voicing difference between /t/ and /m/).

> Consider "inpainting", "unbiased", and (as suggested earlier) "government", each of which is a synchronically transparent /n/ across a morpheme boundary

I don't think "government" is a valid example, and you should stop trying to lean on it. In my view, the pronunciation of "government" has as much to do with the morphemes suggested by its spelling as the pronunciation of "comfortable" does with the morphemes suggested by its spelling.

I have no problem with "unbiased"; that's a great example of what we're talking about.

> Clearly you can see the unnaturalness of lack of assimilation to call the attempt to do so "special effort"!

I don't agree with this. I claim that it is common for Anglophones pronouncing "unbiased" to make contact between the tip of their tongue and their alveolar ridge while they pass over the /n/ in the word. (And here, we're on firm ground saying that the internal phoneme is /n/ and not /m/, since it's part of a productive prefix un-.) I further believe that they make no special effort to do so. They may or may not allow a longer duration of nasal murmur than they do in other contexts, to make the /n/ clear; doing this would constitute a special effort. I believe that some speakers will do this and some won't bother. Of those who do, only a small amount of effort will be given to the task.

But the case of English speakers attempting to pronounce Japanese is different. They will go to great lengths to demonstrate that they want to comply with the bizarre textual representation they see. They are happy to produce highly unnatural speech in order to do so. (Which isn't really a problem; they don't really have an alternative to producing unnatural-sounding speech in early attempts to pronounce a foreign language. But this is something they shouldn't encounter problems with.)

> And in a wider context, the principal users of romaji Japanese aren't anglophones; they're Japanese-speakers who for some or other reason need need to coerce Japanese text into an ~ASCII-subset representation, targeting primarily computer systems with that sort of limitation (most common case being keyboards via IME, hold that thought)

> (This is also why pinyin can be the way it is, yet their IMEs have routinely have modes to ignore s-sh/n-ng/n-l distinctions.)

This isn't a flattering comparison for the all-n Japanese transcription system. The pinyin for 吕 is lü. Chinese people don't use German keyboards, which makes the pinyin impossible to type. So where ü contrasts with u, pinyin input methods require you to input V. And Chinese people have responded to this by adopting v-based spellings; it is common to see pseudo-pinyin like "lv" where that pinyin has been generated by an ordinary Chinese person for their own purposes, such as a sign over their business or an online username.

But the letter V is formally not a part of pinyin at all, which means that text generated by the government never uses it and neither do instructional texts.

It is true that this situation is the reverse of the one we're discussing - the Chinese are making a distinction that is required by their language but forbidden by their keyboard, and the fact that they are aware of the distinction makes it easy for them to know what to do. The Japanese are failing to make a distinction that doesn't exist in their language but does exist on their keyboard; this is precisely parallel to the pinyin IME settings you note that will allow the user to ignore phonemic distinctions that they don't make. Again we see that the system maintains the distinction and it's the job of the input method to interpret what the user wants to say.

Chinese IMEs also offer a "double pinyin" input method, in which you type one letter to indicate the onset of a syllable and a second letter to indicate the rime. All syllables are two input-letters long; this model matches the traditional Chinese view of their own phonology. You could just as easily base your system of English transcription on this: instead of "Xi Jinping", 习近平's name would be "Xi Jnp;". Instead of "Sun Yat-sen", we'd talk about "Sp Yixm".

That's what it looks like when you base spelling on what it's convenient for foreigners to type as an intermediate input to their own, different spelling. (As is the case with Japanese input methods.) There are zero people who believe it's a good idea. It's not a better idea in the Japanese case.