Krashen - Input vs Noise
In his book Principles and Practice in Second Language Acquisition, Krashen discusses what makes a text Comprehensible and has an interesting story illustrating input vs what he terms "noise" that I thought was interesting, about him not acquiring his parents' native language. I think this is relevant to a lot of repeat questions here about "passive" listening and how much it counts. Pablo clearly states that we should be paying attention to and engaged with our input, and this squares with Krashen. I'm including the whole few paragraphs for context, but the paragraph in question if paragraph 4.
This passage also explains the real value of SB material in addressing us as viewer in a way that TV does not.
- OPTIMAL INPUT IS COMPREHENSIBLE
This is clearly the most important input characteristic. It amounts to the claim that when the acquirer does not understand the message, there will be no acquisition. In other words, incomprehensible input, or "noise", will not help.
Positing comprehensibility as a fundamental and necessary (but not sufficient) requirement makes several predictions that appear to be correct. It explains why it is practically impossible for someone to acquire a second or foreign language merely by listening to the radio, unless the acquirer speaks a very closely related language. A monolingual English speaker, for example, hearing Polish on the radio, would acquire nothing because the input would be only "noise".
This requirement also explains the apparent failure of educational TV programs to teach foreign languages. The input is simply not comprehensible. My own children watched programs such as Ville Allegre faithfully for years, and acquired about as much as I did: They could count from one to ten in Spanish and recognize a few words such as casa and mesa. The comprehensibility requirement predicts that TV would, in general, be somewhat more successful than radio as a language teacher, but that even TV would be inadequate in beginning stages. Ervin-Tripp (1973) has noted that hearing children of deaf parents do not acquire language from TV or radio, an observation consistent with the requirement.
This characteristic also explains why children sometimes fail to pick up family languages. My own case is, I think, quite typical. My parents spoke Yiddish around the house for years, occasionally to each other (to tell secrets), and constantly to my grandparents. Nevertheless, my sister and I failed to acquire Yiddish, with the exception of a few phrases and routines. On the other hand, in many families children do grow up speaking the family language as well as the language of the community. What appears to be crucial is whether the family language is directed at the child, in other words, whether an attempt is made to make the language comprehensible. What we heard via eavesdropping was not comprehensible. It dealt with topics that were not easily identified and that were also often beyond our range of experience. Language directed at us in Yiddish would have been simplified, and more relevant to us, and hence more comprehensible.
Another prediction that the comprehensibility requirement makes is that "just talking", or "free conversation", is not language teaching. In other words, simply being a native speaker of a language does not in of itself qualify one as a teacher of that language. Conscious and extensive knowledge of grammar does not make one a language teacher either. Rather, the defining characteristic of a good teacher is someone who can make input comprehensible to a non-native speaker, regardless of his or her level of competence in the target language. This leads naturally to another topic, how teachers make input comprehensible.
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This entire text is available for free on Krashen's personal website: https://sdkrashen.com/content/books/principles_and_practice.pdf